Can't Playback Audio Channels from Timeline

I was given a project to work on in Final Cut 6 that has 6 audio channels. The first 4 work fine. I can play 5 and 6 in preview but I can't hear it when I playback from the timeline. When I open the audio mixer I can see all channels but the last two are set to -inf and I can't change them.

How do I change these settings?

*I hope this isn't a stupid question. It seems like there would be an obvious answer.