Prioritize SIP / voice traffic with QoS / traffic-shaping on the ASA

The configuration following demonstrates how you can shape traffic on the outside interface - limiting it to 5mbps for the outside interface - although ensuring SIP traffic is being prioritized:

priority-queue outside
queue-limit 2048
tx-ring-limit 256

class-map Voice
match dscp ef sip rtp
exit

policy-map p1_priority
class Voice
priority
exit
exit

policy-map p1_shape
class class-default
shape average 5000000
service-policy p1_priority
exit
exit

Note: 'class-default' defines everything - except any other traffic matched in the policy.