By default, when you're on a call in Lync, and you get a second incoming call, you will get both a toast and a ‘pip' sound played to alert you to the second incoming call. Many people find this ‘pip' off putting, and are quite happy with just a toast. Here is how to change that sound or turn it off

Unfortunately I can't take a screenshot of a sound, but here is what a second incoming call looks like

To turn off the 2nd call sound, head to Tools, Options

In Options head to Ringtones and Sounds.

Then click Sound Settings in the bottom right. In the Sound window, find Second Incoming Call.

From here you can now choose an alternate sound, or none at all.

Now when a second call comes in, the user will get a toast and tray notification, but no sound played.