The format iPhone voice memos and most call recorders use. Drop it in as it is — no converting to MP3 first.
M4A is what an iPhone Voice Memo, a Mac recording and half the call recorders on Android produce. It's an MP4 container carrying AAC audio, and it's the file most people find themselves holding when they want something transcribed.
It's also the format most online converters refuse, which is why «m4a to mp3» is such a common search. You don't need that step here: we read it directly, at whatever bitrate and sample rate your phone chose.
Share it from your phone or drag it in from your computer.
With language detection and speakers separated if there are several.
DOCX, PDF, SRT, CSV or JSON, with your fixes included.
The classic: an idea, an interview or a class recorded in the Memos app.
Most call recorders write M4A. Straight in, without a middle step.
Two hours of class in one file, transcribed with timestamps to find the part you need.
Record a voice memo on an iPhone and you get an M4A. Record with most Android recorder apps and you get one too. It's the usual wrapper for AAC audio, the codec phones use because it sounds better than MP3 at the same size.
Upload it as it is: there's no need to convert it to MP3 first, or rename the extension, or anything else. Converting would only shave off a little quality on the way, since every compression works on what the previous one left behind.
Apps that record calls usually hand you an M4A, and they come with a catch: quite often each side of the conversation arrives at a very different volume. If you have the choice, record on speaker with the phone set down; the signal is worse but far more even, and speaker separation gets it right more often.
For field interviews, the thing that changes the result most is getting the microphone closer. An M4A recorded thirty centimetres away comes out clean; the same one recorded from across the table brings the whole café with it, and that is what turns into words nobody said.
Upload it and you'll get the transcript with timestamps, the speakers separated by name and a summary with the key points. You can start without an account.