In my Windows 7, the sound works for some programs sometimes but not for other, and for those other programs, sometimes it works, sometimes not, with the change happening without reboot. Here:
if I click on System Sounds slider, it shows the green bar and gives the sound. If I click on Firefox or Remote Desktop slider, nothing happens: no green bar shown, no sound played. And no sound played from those two programs.
Sometimes it works, sometimes not: some hours ago it worked, more hours ago it didn't, now doesn't again. No reboot or device changes since that.
What is more, the Remote Desktop sometimes appears on this dialog and sometimes does not.
I suspect the changes are connected with logging in and out of Remote Desktop. I did not plug or unplug any devices. I noted that Remote Desktop appeared on this dialogue after the computer went to sleep with Remote Desktop connected, then after it woke up and automatically re-connected Remote Desktop, it appeared on the dialogue. Before this, Firefox appeared, but the sound did not work, but Remote Desktop did not appear. And at some moment before, sound in both Firefox and Remote Desktop worked. No reboot since that time, no devices connected or disconnected, though Remote Desktop loses connection and re-connects periodically.
How to fix this? How to force a program to appear on this dialogue? How to make it react on a click on the slider?