Firstly to have your voice & data on the same subnet is a really bad design. They really should be seperate subnets - as in this situation, it would be so much easier.
If you are unable to change the IP subnets, then you need to mark the voice traffic to then prioritise on the ASA. If you can mark the ToS on the voice then the below config example is all you need.
http://www.cisco.com/en/US/products/ps6120/products_configuration_example09186a008080dfa7.shtml
The only other option is write ACL's that look for specific TCP/UDP signaling voice traffic - which is not the easiest way.
HTH>