I have a computer that should run continuously without any connection to a monitor. For developmental purposes I have been keeping the vga connection with the monitor and experienced no problem until now.

When I start the machine removing the vga connection beforehand, external microphone does not work. At first I didn't know anywhere to look and see the problem, but after a google search I saw that there is a command as alsamixer

I ssh the machine end type alsamixer when it is connected to the monitor, here is the result

If I remove monitor connection and reboot again, and then type alsamixer, I see the error,

$ alsamixer 
cannot open mixer: No such file or directory

I suspect that this error is related to X somehow. I really don't know anything about what goes beyond. This machine needs to work without any connection to a monitor.

I would deeply appreciate any suggestions.