Unfortunately there is no default priority for DSCP/ToS (or for anything else), you have to create your own queues. As default Ros treats everything at the highest level of priority. Basically you have to create a queue for DSCP/ToS and lower everything else.
General info:
http://wiki.mikrotik.com/wiki/DSCP_based_QoS_with_HTB
http://wiki.mikrotik.com/wiki/Manual:WMM#Priority_from_DSCP
http://mum.mikrotik.com/presentations/PL10/grittini.pdf
Examples DSCP/Voip
http://wiki.mikrotik.com/wiki/Voip
http://forum.mikrotik.com/t/prioritize-voip-post-your-ideas/57468/1
http://forum.mikrotik.com/t/qos-for-tos-dscp-rule-check-and-any-suggestions/38781/1
http://forum.mikrotik.com/t/dscp-mangle-and-queue-trees/100297/1