Fmod, while free, is not open source, and is a binary-only distribution. Back in the days of BF, I did actually switch over to fmod for a while, but we found the number of problems that cropped up to be too much of an issue, especially on various linux distros. If anything, should be looking at openal (as assaultcube uses), because it is open source and much more liberally licensed.
These days anything that is free only for non-commercial use is not allowed in RE, as I made an executive decision that everything in the package should be free for ANY purpose, the only caveats I really allow are BY (must give credit) and to a lesser extent SA (share-alike, must share the content and any modifications you make - this one can only apply to art/etc and not code). This allows myself and anyone else to create commercial mods if they want, and thus generate revenue like I hope to one day with another gameplay genre I have planned (when I actually get around to it, lol).