The ‘strange sound’ when the app is in the background is caused by the StudioSystem::update() not being called enough, this is because when the app is in the background Unity limits the app to 1fps and this is what drives Monobehavior::update() which is used to call StudioSystem::update().
If the game is still running in the bacground and you call mixerSuspend(), the game can still make calls to FMOD but they will not be processed until mixerResume() is called, which will result in it processing all the queued commands at once which could also cause ‘strange sounds’.