Have you tried unmuting [Speaker]?
Separately try different settings of [Auto-Mute]
Have a play around with alsamixer, sound issues do not always respond to logical diagnosis, more trial and error.
You could check Software Store if you have Pulse Audio Volume Control (pavucontrol) installed. That is another tool that may help solve the weird behaviour. See this post by @Aravisian Sound doesn't work on Acer Gateway NE46Rs laptop - #5 by Aravisian
Also any answer to this question:
Another thing to think about is linux kernel version and support for your audio hardware. Can you tell us the make/model/spec of your computer?
If it is very old or very new machine the linux kernel version could be a factor.