Channel mapping for multichannel AAC?

[Oobleck]

Hi all! I’m experimenting with converting multichannel FLACs to 256 kbps AAC (with an eye towards uploading them to Apple Music; apparently Apple Music will not match the file, and will stream in multichannel).

I drop the FLAC into Audacity, in this case a 4.0 file) then export to AAC. But the channel mapping is off when I use the default mapping…entire channels drop out and it’s just not right.

Anyone know what channel mapping AAC uses? How can I accurately map the FLAC channels to AAC?

Many thanks!