ASA(config)# priority-queue outside
ASA(config-priority-queue)# exit

ASA(config)#access-list VoIP-Traffic-OUT extended permit tcp 192.168.1.0 255.255.255.0 any eq sip

ASA(config)# class-map Voice-OUT
ASA(config-cmap)# match access-list VoIP-Traffic-OUT
ASA(config-cmap)# exit

ASA(config)# policy-map VoicePolicy
ASA(config-pmap)# class Voice-OUT
ASA(config-pmap-c)# priority
ASA(config-pmap-c)# exit

ASA(config)# service-policy VoicePolicy interface outside 

show priority-queue statistics

source:http://www.networkstraining.com/cisco-asa-qos-for-voip-traffic/