When they play that song...
Is that supposed to be diegetic? Like, to put it in simpler terms, is that what they are supposed to be actually playing, or is that just a song that only we the viewers can hear? Because there's singing in the song, and neither Bill nor Ted are mouthing the words, which leads me to believe perhaps they were playing something different than what we were hearing...
***For your information, everyone knows monsters prefer blondes.***