I have a couple CX100s I have used for years (even before going Polycom) with OCS and now Lync and have always had excellent audio quailty. If you are seeing this only on a single unit then I'd guess that the device may physically be at fault.
Can you test by setting it as the default computer audio device and see if all audio playback (not just Lync sounds/calls) is affected?
Jeff Schertz - Principal Microsoft Solutions Architect [Blog]Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels.