Richard,
You have to configure the policy for the QoS on the ASA. Firstly are you marking the RTP traffic anywhere in your network - as this would make things much easier, DSCP values? If not - what port range are you using for RTP, as you can configure a policy based on this port range? If you don't know - you can use a source/destination acl to identify the RTP traffic? If you have VPN tunnels - you can use these?
something like:-
access-list inside_QoS line 1 extended permit ip x.x.x.x y.y.y.y z.z.z.z w.w.w.w
class-map inside-QoS-example
match access-list inside_QoS
policy-map Inside_QoS_policy
class inside-QoS-example
police input 8000 1500 conform-action transmit exceed-action drop
service-policy Inside_QoS_policy interface inside
HTH.