OSS stands for Open Sound System, and is the interface to which sound drivers were once written to conform.
ALSA (Advanced Linux Sound Architecture) replaces OSS, and allows more flexibility than OSS.
OSS will be removed from the kernel in some future version, probably either 2.8 or 3.0. But it sounds like you should be using the OSS compatibility layer built into ALSA, which will probably stick around for a while yet.
sox is a sound-processing utility, and it's a little worrying that it's not upgraded to ALSA yet. Firefox is a web browser, and it can probably use other sound systems anyway.
I shouldn't be too concerned because by the time you've upgraded your kernel that far, you will (hopefully) have upgraded sox and firefox along with it.
Being unable to play MIDI sounds is a driver issue; you may simply not be loading the driver for your MIDI chipset. This tends to be hard to autodetect, so it often has to be set explicitly.
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