How UDP jitter help me to monitor VOIP quality??!

I am wondering how UDP jitter helps me to monitor the call quality in the network??

We have a MPLS network and using QoS on it. Voice traffic is using priority queue, but the QoS is marked based on IP access-list. (router IPs are in the default class)

I dont want to add my router IPs  to the QoS priority class, becuase I need to monitor the defualt class performance also.

And now, I am using UDP jitter operation to monitor the WAN quality.

My questions are,

1) the UDP jitter operation is monitoring only the default class of the MPLS ?

2) the voice calls are running on priority class, how can i use IPSLA to monitor the jitter, latency and packet drop ,etc?

BTW, for UDP jitter operation, there will be source and target? How can i define which one is source and which one is target?

the router with "ip sla responder" is the source or target?

Thanks in advance!!