I was messing around with my own PCB recently and noticed the background tune during gameplay. Checking MAME out, the tune appears to be completely missing as far as I can tell (I've tested all of the ROM sets)
As Pleiades is a pseudo-sequel to Phoenix (which had its own little tune) , it should come as no surprise that this game also has a similar ditty playing in the background.
Perhaps if someone can confirm this, I can add the bug.