If you’ve never discovered the wonders of Soundflower, you are truly missing out. Soundflower allows you to route audio within your Mac with unparalleled simplicity, and is a free download from the guys behind Max/MSP (Cycling 74).
Unfortunately there is a rather annoying glitch where horrible noise starts to develop after about 5 minutes of use. (It starts out starts to make things sound like Darth Vader.) This makes it practically unusable for any live or extended use, and has been recognized on the Google Code site.
You can go read through the thread and find out for yourself, but here’s the workaround, in Snow Leopard.
1. Create an Aggregate Audio Device in Audio MIDI Setup (Applications > Utilities > Audio MIDI Setup) by hitting the Plus (+) button.
2. Add Soundflower and your main output (in this case Built-In Output) to the Aggregate Audio Device by checking their “Use” boxes.
3. Set the Clock Source to “Soundflower”.
This will make sure that Soundflower is governing the timing of your samples, which seems to prevent it from tripping up. Good luck!