MusicBee by default uses the Windows native codecs/foundation for decoding AAC files, unless the bass_aac.dll is added by the end user to the MB directory.
Some people have reported they can't play certain AAC files in MB and that adding the bass_aac.dll fixed that. I've often wondered why that was, and just recently found that while AAC supports a sample rate of 8 to 96kHz, Windows only supports up to 48kHz. As bass_aac.dll does support sample rates up to 96kHz, that explains why it's needed in MusicBee if you have any M4A/AAC files at that rate.
Just FYI because I don't like "mysteries", haha.. If you do need the bass_aac.dll check news about where to get the latest version here:
https://getmusicbee.com/forum/index.php?topic=23454.0 .
[modified 12/15/2018]
Topic: AAC sample rate and MusicBee (Read 3421 times)