How to trim an MP3 file
Cutting audio precisely needs to see the waveform, which most built-in apps do not show. Here is what actually works.
Why this is fiddlier than trimming a video
Most phones show a visible timeline with a video preview when trimming, which makes it easy to see roughly where to cut. Audio has no picture, only sound, so trimming accurately without seeing a waveform means guessing based on playback position alone, which is slow and imprecise for anything more exact than "somewhere near the start."
On an iPhone
The Voice Memos app can trim a recording it made itself: tap the recording, tap the options icon, Edit Recording, then drag the trim handles at the top of the waveform. This only works for recordings made in Voice Memos, not for an MP3 or other audio file already on the phone from somewhere else.
On Android
There is no universal built-in audio trimmer across Android phones; some manufacturers' own Sound Recorder apps include one for their own recordings, similar to Voice Memos, but it doesn't handle an arbitrary MP3 file already saved to the phone.
On a Mac with GarageBand
GarageBand can trim any audio file: import it as a track, drag its edges to the length you want, and export with Share, Export Song to Disk. This works for any MP3, not just recordings made in the app, but it is a full music app for a simple cut, and exporting adds a few extra steps compared to a purpose-built trimmer.
A waveform-based trimmer built for exactly this
Stayput's trim audio tool shows the waveform so you can see exactly where to cut, works on any MP3, WAV, M4A or other audio file (or the sound from a video), adds fades if you want them, and exports as MP3 or WAV, all in the browser with nothing uploaded. It works the same way on a phone or a computer, unlike Voice Memos or GarageBand which each only cover one device.
Questions
Can I trim an MP3 without installing an app?
Yes, a browser-based trimmer with a visible waveform works on a phone or computer without installing anything, and handles any MP3 rather than only recordings made in a specific app.
Why do I need to see a waveform to trim audio accurately?
A waveform shows where the sound actually starts and stops, including silences and peaks, which makes it possible to cut precisely instead of guessing from playback position alone.
Does trimming an MP3 reduce its quality?
Trimming itself doesn't re-encode the audio content, only shortens it; quality loss only happens if the tool re-compresses the file at a lower bitrate during export.
Can I add a fade-in or fade-out when trimming?
Some tools support this, including Stayput's trim audio tool; a plain trim without a fade can sound abrupt at the cut points, especially for music or speech.