...should we recommend that new audio contributors would leave some contact info, an email address for example, on their forum profiles? This way we would have some way of contacting them, if for example our licenses change, or it isn`t clear if they agree to publish their work in FreeOrion? Or do we already have this contact info, since registration to the forums requires a working email address?
Forum admins have access to the email address used to register an account, so we don't need another as long as the registration one isn't a throwaway / unmonitored spambucket.
I'd rather not need to send such an email though...
We could also recommend that they would state clearly when they post a track or a link to a track on FreeOrion`s forum, if they approve that the track can be used in FreeOrion
Having submitters note explicitly that they release their contributions under the CC-BY-SA 3.0 would be best. They don't need to specifically mention FreeOrion in such a note, and indeed doing so makes no difference, since anything released under a compatible license can be used by us, and anything used by us must be released under a compatible license.