synchronizing audio tracks

:question: I have 2 audiotracks to merge, one is a solo track of a lead singer and the other is the entire choir.
the song is from a musical and does not follow a strict tempo.
The problem is that the files are difficult to get in to sync because the recording from the choir is partially slower than the recorded lead singer. My question: what is the best way to get these files in sync and with what tool(s) in Cubase Pro 8??