Well, in general you could use mangle to get UDP port 5060 for the SIP, you could connection mark it, and then prioritize based on the conn. mark - that's what I do, except I use my source IP / dest IP and only prioritize my own VOIP services (not others ).
You could mark just SIP and prioritize the whole connection, if you're lucky the RTP and SIP will have the same destination IP. I don't do it that way, but you should be able to tell immediately if it's marking properly, you'll see that queue tree entry bps increase. If it goes up a little, you only got SIP, if it goes up a lot, you got the audio stream as well.
I know my VOIP over 802.11b to Mikrotik works great with queuing, I get about 2.3Mbps (actual speed) out of my connection using many downloads, I can max it out while talking on the phone and not lose a single bit of audio.