When System 6 (UNIX) was released, Andrew Tanenbaum created an OS called Minix. Minix was very much like UNIX, but designed as a teaching aid. Linus came along and demanded more from Minix, he added to it and gradually replaced the Minix code, creating Linux. (very brief history)
As Nick said, OS X takes parts from BSD. It doesn't (shouldn't?) contain any of the original UNIX code (as the code was released under different licenses).
Today UNIX is more of a standard, OS X (10.5 and up) is UNIX certified. The standard specifies programming interfaces (header files, etc), system calls and utilities (sh/vi/ed/awk/etc). I'm not quite sure how it differs from POSIX (an early version of Windows was POSIX compliant). The spec doesn't require any original UNIX code, so theoretically, MS could re-giggle Windows and make it UNIX compliant...
Because of the Minix link, Linux feels a lot like UNIX, they are designed similarly (but not the same).
There we go, I'm pretty sure that's why people turn me down when I invite them out for a quiet pint
If you have any questions, I'll happily send you a couple of PM's, I may bore you, but I'd have great fun

Ideally, you want to keep /home (and /var really) on separate partitions. If you need to combine, as the others have suggested, use GParted, or if you looking at reinstalling then do it then at install time.
As to the sound, why are you using Xubuntu as apposed to Kubuntu? If Kubuntu works and does everything you want, then I'd look at that (if I'm talking crazy, I apologise, I'm not sure of things when it comes to Linux)
Ben