Sorry for the late reply, but I wanted to make sure I am accurate before I post a response.
The song that plays when only Ayumi or Yoshiki is alive is called "Light to Despair". It is only found on the Corpse Party BloodCovered; Sound Collection Vol. 2 CD as track 24. Unfortunately, there isn't a place to hear it online.
Yo Enig wanted to tell you I don't care about what happened to Alex anymore.I actually stopped caring the day one hour later. He might have my name but he isn't me. Just saying in case you were worried. Oh yeah I wanted to ask you if you knew what the song for when in Corpse Party PC version. You know the one when Ayumi or Yoshiki die in the science room. By any chance do you know where I can hear it?