If you are looking for free to use music to use in your (non-comerical) podcasts, Magnatune is a music download site that allows you to use its music in your podcasts for free.
Also look at opsound which distributes music under a Creative Commons Attribution-ShareAlike licence. You would just need to attribute the work to the creator and your work would have to be distributed using the same licence.
Using free software like Audacity you can easily edit and combine your recordings with music. To load music into an Audacity project you would select the 'Project' menu and then 'Import audio...'.
If you really wanted to create your own music, the GarageBand software on the Macs upstairs in the LINC building makes it very simple.