I couldn't find a way to do this with Soundbooth. I know with Audition you can mark one or more tracks to record as others playback, but Soundbooth seems to have separated the recording function from the multi-track system.
You might want to look at a demo of Audition 3. I have the Master suite and still bought Audition separately to handle my sound work, because I just don't feel Soundbooth (at least in CS3 and CS4) is up to doing much more than your basic wedding video type of work.
You might also consider that a fairly good multitrack mixer board can be had for around $100 (less depending on the number of inputs) and these give the option of doing what you are talking about -playing a track out plus recording in to a final mix.
Of course, if you are looking to keep the tracks separate for later editing mixing and you just need to hear the audio track for a baseline then Audition is going to be the only solution that I know will keep it separate.