Hotkey to Mute MiSTer?
So I just connected a stereo box through rca - p2 from mister to it but when I want to change a game or reset it does a loud annoying "THUMP!"
so everytime I have to turn off the stereo box to avoid it so I thought: is there a way to create a hotkey for that global volume to be turned off so I can chanage game or reset mister and then press it and it goes back to the level that was previosuly (my case 4 bars)? 
As far as I know, the sound output comes directly from the FPGA, so when the FPGA gets reconfigured to become a different hardware, the sound output is not driven. Hence the noise.
In order to prevent this, the part of the system that is kept alive in between core changes should take control of the audio. That part is the linux subsystem. But, as far as I know, there is no physical connection between those two parts of the MiSTer.
You might be able to dodge the problem by adding an I/O board, if you don't already have one, and hauling the sound digitally via a Toslink cable. From jotego's comment, It sounds like what you're getting is a physical analog pop from circuits disconnecting internally. Using digital sound to a DAC or a receiver should avoid the problem completely.
Another way to reduce the effect would be by setting the Mister volume to max, and then turning the volume down on the speakers. This would make the thump much, much quieter. Right now, you have too-quiet Mister sound that you're over-amplifying, so the thump is super noticeable. Turning the Mister up will make sound you want louder, and then turning the speaker volume down will make the sound you don't want, quieter.
The "thump" is pretty bad even with Mister volume maxed. It is zero issue on the digital side though, whether toslink or from HDMI. You can use a mini-toslink to SPDIF cable with a cheap USB-powered DAC from Amazon that will be miles better than the onboard analog and solve the issue. (or of course, SPDIF directly to your receiver if it can take it.) Mister also has a 96khz audio option that works well so keep that in mind when picking a DAC.
My logitech keyboard has multimedia keys that are supported by the MiSTer. Holding the FN (function) button + F10 mutes global audio and FN + F12 unmutes it.