As always it depends. When you say sound quality what is the problem?
Head end clipping?
Music on hold quality problems?
What is the size of your link?
Are you using the same link for VoIP traffic as data traffic?
Echo is a different problem. If your equipment vendor complies with G.165 or G.168 Echo cancellation just tune the echo tail as high as you can go.
As far as sounding underwater, you probably need some sort of queuing technique to prioritize voice traffic. Since you also have data traffic riding on that link you may need a way of breaking up the data packets also. You should ask your vendor what they can do along this line.
BTW…throwing bandwidth at the problem can help in the above scenario.