M4A to MP3
Convert an M4A (AAC in an MP4 audio container) to MP3. Both are lossy, so this is a second lossy encode on top of the first: some quality that AAC already discarded stays gone, and MP3 discards more of its own.
M4A to MP3 unpacks the AAC audio sealed inside an M4A container and encodes it fresh as MP3 through a LAME encoder loaded on demand. Both codecs are lossy, so this stacks a second round of loss on top of whatever AAC already discarded. WebCodecs does the decode, so a browser tab is required to open this page.
Questions
Am I compressing twice?
Yes. M4A holds AAC, which is already lossy, so converting to MP3 decodes what AAC already reduced and re-encodes it again through MP3 own lossy compression. Some quality AAC discarded is gone for good, and MP3 discards more of its own.
What controls the result?
The quality option, low, medium or high, medium by default, the same three-preset control every lossy target in this pipeline uses. There is no bitrate field.
Which engine handles this, and where can it run?
mediabunny and WebCodecs decode the AAC, and mediabunny loads a LAME-based MP3 extension on demand as a separate chunk to encode the result. This is browser only; the command line and MCP refuse it and point to the web page.
What size should I expect?
Usually a similar order of magnitude to the source M4A, sometimes a little larger or smaller depending on how the two lossy codecs compare at the chosen quality.
What is the reverse page?
There is no dedicated mp3-to-m4a page. Convert audio, the hub, can write m4a from an MP3 source directly, using the same AAC-in-MP4 container m4a-to-wav reads.