One thing to keep in mind, with regards to sounds, you may be alienating (no pun intended) your mobile userbase (which for me makes up about 75% of my users); I for one do not go back to sites which play sounds on my mobile device, even if its a transitional noise. They're annoying, encourage me to turn my sounds down, play over my Bluetooth speakers, and play over my music.
As for placement of coding, this goes for both of your threads here; you don't have to modify everything just because you can. Items at the top and bottom of UBB.threads (and meta/script/style tags) can all be inserted through HTML Includes, and CSS entries should be added to your styles (or at the very least, an HTML include). Placing coding in these open areas allows you way easier management of these tags when changes need to occur; vs having to tear through files at a later date.
As for "how to" on injecting 3rd party code into UBB.threads, other than pointing to the location where you should be placing data, modifying the base code is beyond the scope of this site; modifications to the base code are not supported in any way, and modified forums do not qualify for helpdesk support.
