Are there any softphones that do as the title asks?
We've been using 3CX and I've tried Blink, EKiga, X-Lite and a few more which I have forgotten the name of. I can't seem to find any softphone that supports the windows "Mute all other sounds" Or the reduction to 80% etc that you find on the communication tab. The only program I know that does this right is Skype, but skype can't connect to SIP (without an extra service that they charge for). My boss isn't against buying a license for a product, but I don't think he'd go for a contract or per-use fee based product.
I thought X-Lite did the job, but it turns out it only pauses the media player. This wouldn't be that bad except if you have audio coming out of a browser. It doesn't deal with that.