Are you using a communication device?
There is a setting to allow other sound producers to control their own volumes. If that is set and that sound happens, it migh be louder or softer that you think it should be.
To make sure, if you right click the speaker icon then playback devices, select the speakers and click on properties, then the levels tab, you will see a number. Make note of what it is and when you think the level has changed, go back and see what it changed to.
If you want to keep other utilities from having separate volume controls, right click on the speaker icon and choose Volume Control Options. Check or uncheck options to test.
Are you running any other mixer?