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.