interesting tell of tales here...
I was telling Broodwich that I never saw (or heard) the attract screens from the original MK3 cause when MK3 was in circulation in arcades it was busy being played constantly, and within a mere few months, those all got turned into UMK3s, which is what I saw in later years.
I've been playing with MAME since 2000, since before MK2 and MK3/UMK3 were even emulated yet for that matter, and as far back as I could remember, MK3 had no attract music, like the OP said, I only remembered the bell gong and nothing more.
so this issue with the UMK3 U2 sound rom being bad and supposedly killing attract sounds or whatever really piqued my interest and when I loaded MK3 in my 64-bit MAME, I heard music finally. so what the hell happened here? I kept all my old versions of MAME, cause I had certain romsets that I had to replace in newer versions and never got around to it, blah blah... and I dug up my ancient MAME32 and loaded MK3 and sure enough, no music again.
one thing I observed though was that it was asking for the umk3-u2.bin file. for whatever reason the MAME guys fucked up back in the day and were reading from the wrong U2 rom file. lol
just thought I'd share this, seeing as how I never knew where that music from the MK3 soundtrack CD came from.
now simply because I thought everyone was full of shit about the UMK3 U2 rom thing, I copied my mk3-u2.bin to the UMK3 set and oddly, all the attract music still plays. I tried to reproduce the glitch in MAME, but it cycled through the attract music twice and went to silent like it normally should, it never cut off in the middle or anything. or is that what it does? I'm not entirely sure what rom my physical UMK3 board has.