Thanks to Koto and Soneek for the loop points. I batched this in a program to append loopstart and loopend to the beginning of the looping AAC. With that I was able to use VGMToolbox to batch the songs into Genh. The looping sounded a bit off, with clicking, so I loaded a new loop into audacity, with an un-modified version. I noticed there was a 1024 sample offset, so each loopstart and loopend posted in this thread had 1024 subtracted for this set. The music now loops seamlessly. This should work in the latest build of droidsound-e. There was a bugfix to allow these all to play. Download Here I'm not sure if this might be a better version to have on 3ds.josh.info?
All songs that are still in AAC format did not have any loop data, and are unmodified.
Reminder for those creating GENH/TXTH + AAC, is this isn't super obvious.
MP3-like formats (AAC/ATRAC3/OGG/etc) often need "skip samples" (aka encoder delay = samples to ignore at the beginning) to get gapless/accurate decoding.
The value depends on the encoder and header but for AAC is typically 2112 or maybe 1024 (experiment). For GENH there is a setting at the bottom of the latest VGMToolbox; for TXTH just write "skip_samples = 2112".
Now, GENH/TXTH loop values should *not* include skip_samples, so you may need to substract. Ex. if some loop_starts are lower than 2112/1024 then it's probably ok, otherwise check if substracting -2112/1024 loops cleaner. Conversely if you don't set skip_samples you may need to add 2112/1024 to loop values (plus your file will have garbage samples at the start).
Non-looping AAC benefit from GENH/TXTH + skip_samples too, otherwise it's random whether your player supports it.