In Ubuntu, the best/easiest way to install the Broadcom WiFi driver is to install the appropriate software package ("bcmwl-kernel-source") via one of the the package managers (like Synaptic). I expect a similar procedure will work in Linux Mint.
As for the sound driver, it should be included. Maybe sound was muted by default (I think this has affected some people). The next time you run Linux on the Mini 9, open a terminal, and run "alsamixer". It's a simple mixer app that let's you adjust speaker volume, mic volume, etc. If there's no sound driver, you won't see all those options. If a input/output is muted, it'll have an "M" at the bottom of the control. Press "m" to mute/unmute when a control is selected.
And about XP, it should run well on the Mini 9, and there should be a thread somewhere around here listing what drivers are needed and where to get them.