I've noticed that some SIDs play back in Rockbox while some silently fail (the playlist skips to the next playable file), and on closer inspection it appears that Rockbox does not support RSID formatted SIDs. PSID is the aging standard, while RSIDs are used for games which require emulation of more than just the MOS SID chip to play correctly--the most prominent one in my collection would be Turbo Outrun (SID in HVSC), but there are many others. Here's a brief quote from a
tech doc on the various SID formats:
The RSID format was designed to contain tunes that are not PlaySID compatible,
but strictly require a real C64 environment to run. Tunes that are multi-speed
and/or contain samples and/or use additional interrupt sources or do busy
looping will cause older SID emulators to lock up or play very wrongly (if at
all).
By using the name RSID for such rips all existing SID emulators will reject
these tunes safely until they can be upgraded to cope with the additional
requirements.
Due to the nature of these tunes, every effort must be made to make sure they
are directly runnable on an actual C64 computer. As such the tunes will only
be presented with the default C64 power-on environment and expected to
configure and use all hardware appropriately.
RSID is based on PSIDv2NG with the following modifications:
magicID = RSID
version = only 2
loadAddress = 0 (reserved)
playAddress = 0 (reserved)
speed = 0 (reserved)
psidSpecific flag = 0 (reserved)
The above fields MUST be checked and if any differ from the above then the
tune MUST be rejected. The definitions above will force tunes to contain
proper hardware configuration code and install valid interrupt handlers.
I'm unsure if anyone is currently working on SID playback, but if there's any interest in it, this may be an ideal focus point for effort, as it currently prevents a largeish subset of SIDs from playing under Rockbox.
EDIT: Sorry, should have checked Flyspray first; it has been discussed there, but only in the comments of another issue as far as I can tell, it has not been filed individually and is technically not a "bug" per se anyway. Here's the comment where RSIDs come into play:
FS#8287 comment. Since Rockbox's SID playback is apparently based on TinySID, which lacks RSID support, adding playback would probably require a substantial effort or even a complete port of sidplay2.