You're welcome, I really enjoy these old lego games and it was something I never ever realised when I was a kid. Thought it would be an easy fix but clearly not.
I also made another discovery today, a different game with exactly the same issue. Rollcage from Psygnosis for PC. Bought a "new" copy on eBay and installed it. No music at all. Checked the CD files and there isnt even a mention of music anywhere on there so I panicked a little and thought it must be an issue with my computer since Rock Raiders was doing the same thing. After your replies on here, I tried it with a different CD version of the same game (im assuming an older official release?) and it had music on it. Then I thought it might be exaclty the same issue so I put both of the discs into a CD player and as expected, one played music like a CD and the other didn't play anything. So this must have been a common way to get music playing on PC games in that era? This is just something I never would have expected, surely it must be a space-saving solution since the music doesnt even show up as a file on the disk, maybe it tricks the CD into adding more files? Maybe?
Also, yes its weird that two seemingly identical disks are actually different, even with the same serial numbers. I double checked and yes, one has music and one does not. Youd think the publishers would check this before shipping them out but they really must not care too much, at least not as much as the OG developer/publisher. What is crazy to me though is that the collectors "Classics" version of the game indeed has no music, youd think maybe they would have at least checked that one?
Glad I can add to the LRR community in some way!