It looks like it's trying to set the left/right panning balance of a sound channel, and displaying the error code if the operation failed. I'm pretty sure that it is non-fatal failure, at worst maybe the sound wouldn't sound quite right but shouldn't otherwise be interrupting the game with what looks like leftover debug mechanism.
If there is already an existing mechanism in your codebase to direct debug messages to a log file, I'd strongly recommend changing the ShowMessage to output silently to such debug log file instead. Otherwise it's probably safe to just remove the ShowMessage() call in the code snippet.
[edit: maybe I speak too soon. I'm not sure how the access violation mentioned by Wafflem happens, though I'm pretty sure it is not solely dependent on BASS_ChannelSetAttribute(..., BASS_ATTRIB_PAN...) failing. Most likely there is just some portion of sound code, likely not shown in the snippet above, that implicitly assumes some sound-related operation can always succeed, and then behaves badly when it doesn't succeed, for example due to no speakers/audio output devices installed, and eventually the code did something stupid accordingly that causes the access violation.
Removing ShowMessage() would obviously not do anything about the access violation, but at the same time it's not making things worst either.]