A song you have played a hundred times suddenly has a fuzzy edge on the loud parts. Or the speaker crackles during videos but sounds fine on calls. Or everything is fine until you turn it past three quarters, and then it falls apart.
The obvious conclusion is a dying speaker. Usually it is not. Distortion comes from a short list of causes, and you can narrow it down in minutes without tools.
Work through it in order. The first step tells you which of the later ones matters.
Name the noise before you fix it
Different faults sound different, and the noise tells you where to look.
A fuzzy, torn quality on loud passages only is clipping. That is a level problem, not a hardware one, and it goes away when you turn down.
A dry crackle or rattle that comes and goes, often on bass notes, usually means something physical: debris in the mesh, moisture, or a loose case buzzing against the phone. A constant hiss points at interference or a bad connection. Audio that stutters and drops out for a fraction of a second is a wireless problem rather than a speaker one.
The three-swap test
Change one thing at a time. Play a different track: if only one file is bad, the file is bad. Play the same track in another app: if the problem follows the app, it is that app's settings or a bug. Then switch output, speaker to headphones or back: if it follows one output, that hardware is your suspect. Three swaps beat an hour of guessing.
Clipping is the first thing to rule out
Every audio path has a ceiling. Push a signal past it and the tops of the waveform get flattened, which your ear hears as a harsh, torn sound.
It is easy to cause without realising, because gain stacks: loud file, plus a boosted EQ band, plus a boost in the app, plus system volume near maximum. Each stage seems reasonable and the total is not.
The giveaway is that it scales with volume. Quiet is clean, loud is dirty, and the turning point is consistent.
So remove gain rather than add more. Cut the bands you boosted by three or four decibels rather than raising the ones you did not, and lower the boost a notch. In Volume Booster: Bass & EQ that means backing the master level down until the harshness goes. If you are on a bass heavy preset, try a flatter one.
Physical causes that sound like electrical faults
Phone speakers sit behind fine mesh that collects pocket lint, and a partly blocked grille sounds thin and buzzy rather than merely quiet.
Check the slots at the bottom edge and the earpiece above the screen in good light, then clean gently with a soft dry brush. Do not push anything into the openings, and skip compressed air, which forces debris further in.
Moisture is the other common one. Water in a speaker makes a bubbling crackle that fades as it dries, so leave the phone somewhere dry and airy rather than reaching for a hairdryer. Check the obvious too: a thick case or a misaligned screen protector over the speaker cutout changes the sound, so take the case off and listen again.
Bluetooth crackle is its own problem
If distortion only happens on wireless headphones or a car stereo, treat it separately. Bluetooth shares crowded radio space with Wi-Fi, and interference sounds like stuttering, popping or brief dropouts rather than steady fuzz.
Move the phone closer, take it out of a back pocket, and see whether another room helps. Turn Bluetooth off and on. If it persists with one device only, forget that device in Settings and pair it again, which clears a surprising number of odd faults.
Quality also varies with what else the phone is doing, so a heavy download or a crowded café network can show up as choppy wireless audio.
Check the settings that quietly change your sound
A few options make audio sound wrong even when nothing is broken.
In Settings, under Accessibility, then Audio and Visual, there is a left and right balance slider. Nudged even slightly, one side sounds hollow and the stereo image collapses. Mono Audio sits in the same place, as does Headphone Accommodations on supported devices, which applies a tuning curve some people find odd on music.
Under Sounds and Haptics, the Headphone Safety limiter reduces loud passages, which is protection rather than a fault but does change the sound. In the Music settings, Sound Check evens out level differences between tracks, and the EQ presets there can add enough gain to tip a loud master into clipping. Names vary between iOS versions, so look around the section if a label differs.
When it is the source, not the phone
Sometimes the audio was damaged before it reached you. Low bitrate files, ripped videos and heavily compressed streams have a grainy quality no setting will remove, identical on every device.
Streaming apps also drop quality when the connection is poor. If a track sounds rough on cellular and clean on Wi-Fi, that is the network, not your speaker.
Restart the phone and install pending updates before concluding anything. If distortion survives a restart, appears in every app, on speaker and headphones, at every volume, book a hardware check.
Quick answers
Why does my iPhone speaker crackle at high volume?
Usually clipping, meaning the signal is pushed past what the speaker can reproduce cleanly. Lower the volume a couple of notches and reduce any bass boost. If it crackles quietly too, look for debris or moisture in the mesh.
Does a bass booster damage your speaker?
It will not break a phone speaker in normal use, since output is limited, but heavy low frequency boost makes small drivers distort long before they are in danger. Treat the mush as a warning.
Why is only one side of my headphones distorted?
Check the left and right balance slider under Accessibility first, then try the headphones on another device. If the fault follows them, it is the cable or driver. If it stays with the phone, it is a setting or the port.
Should I reset all settings to fix distorted audio?
Only as a last resort, after the swap test, a restart and an iOS update. It clears Wi-Fi passwords and preferences without touching your data, so it is recoverable, but it rarely fixes anything a single wrong toggle did not cause.
Where to get it
When distortion comes from stacked gain, it helps to have level and tone in one place. Volume Booster: Bass & EQ puts a ten band equalizer and a master level side by side for tracks played inside it, so you can hear which change causes the harshness.
Cut first, boost second, and most crackle disappears on its own.