As a general matter if you leave media handles FCP will edit in transitions without changing the duration of your sequence.
I'm not very good with audio, but it seems to me if you have multiple tracks – each specific to a player/instrument – blading is not your friend. (No handles.) Try keyframing (option click on the audio levels line on the tracks) the outgoing track and the incoming track to overlap according to your needs.
What the heck are handles?!?! I keep hearing about using the handles, and this and that. I'm guessing that that is just the thing that you can pull that will lengthen/shorten a clip? I don't know, but the version of FCPX that I have doesn't have the available media option and all. The key framing was the answer, I wasn't aware before that I could key in different effects, too (which was the real problem).
Handles are surplus media on either side of a marked clip, which FCP can use to overlap a cross dissolve without rippling the sequence. For example, a 10 second clip might have an in point marked at 2 seconds and an out point marked at 7 seconds. When edited to the timeline, it's a five second clip with handles of 2 seconds at the start and 3 seconds at the end which are available for transitions (or additional trimming).
Glad the keyframing worked.