Making AAC audio play where MP3 is expected
What AAC is
AAC was standardised in 1997 as the successor to MP3 and is the audio format inside most MP4 videos, iTunes and Apple Music. It is technically the better codec. MP3 is simply the one every device understands.
Why AAC files cause trouble
A plain .aac file from a voice recorder or a video export can fail on older players, car stereos and some Windows apps, or play without any song information. MP3 avoids those problems.
Where these files come from
Screen recorders, call and voice recording apps, some Android apps and video exports often produce AAC audio. If the file ends in .m4a instead, the M4A to MP3 page covers it in more detail.
Protected files are a dead end
Songs bought from iTunes before copy protection was dropped in 2009 can be locked, usually as .m4p, and cannot be converted. Only convert audio you own or have permission to use.
A small quality cost
AAC to MP3 compresses the audio a second time, and MP3 is slightly less efficient than AAC. For speech, lectures and everyday music you are unlikely to notice. Keep the original if the recording matters.
If your file ends in .m4a rather than .aac, use the M4A to MP3 page. It is the same audio codec in a different wrapper.