Hi.
I decided to repost this under a new subject line so that those who are having trouble with the volume slider disappearing could find it easier.
I have discovered something however, that may be of some use to those who are also having trouble with the slider disappearing on their MJ8.
While I still believe the problem to be a bug with MJ, I think there may be some merit to your claims about the sound card issue.
Here is what I have found out:
Whenever my slider disappears, I can bring it back by doing the following:
1) open the volume contol on your system tray and set the volume around half way.
2) Go to the preferences on MJ, click on start up options, enable 'volume control on startup'. Set it to 100%.
3) Close MJ, restart the computer and then restart MJ.
Since every computer is as different as the people who own them, I cannot say for sure this will work for you. However, my slider has disappeared twice now, and I was able to fix it by doing what I just described.
Good luck.