So I tried to look at RAD Audio. The main problem is that its binary specification is unclear (i.e. decompilers have problems with it) and it’s not something easily approachable without one.
Anyway, from what I managed to figure out, it’s more or less equivalent to AAC-LC except being less extensible (and thus slightly saner).
While AAC-LC gets packed into some container format with excessive features (even something like ADTS), here packet header combines all information: first byte is packet header byte (0x55) followed by a byte or two of packet flags (i.e. what features are enabled and if following number fields are 8- or 16-bit values), packet size (8- or 16-bit), two other numbers with unknown meaning (also 8- or 16-bit each, they might be related to the frame coding parameters like number of subbands in use).
Frame structure from a quick glance looks very AAC-LC as well, with active use of Huffman codebooks for coding most of the data (initially I found just two codebooks, apparently the actual number is closer to twenty). And there are tables describing coding modes for four principal sampling frequencies (24/32/44/48 kHz) and two coding modes (normal with mixed short and long frames, and short frames only mode; normal mode is remarkable for having a special frequency table with the same number of bands as for short frame but designed for a long frame—I suppose it’s for the transition frames).
Overall, it’s a rather simple audio codec in a rather simple audio format (and for that reason I’m not interested to look at it further—figuring out details is tedious and serves no real purpose for me). And for the next thing, there’s apparently some fruity codec used in proprietary VNC protocol extension that’s worth looking at…