MOSFET to switch audio speaker signal on/off?

I would suggest you at least start with a typical Relay Module.

Its a low risk option so you won't have to troubleshoot a more high-tech design. And you may find you can wrap it in some sound deadening material and not have a noise issue.
Else you can later on add a solid state higher tech solution. But at that point you will have a working solution as a baseline.

Reed relays might work but be careful the reed switches in these relays are not very robust. The contacts can stick (actually weld closed) with not a lot of power. When working with electronics there are two criteria:

  1. the static requirements i.e. 100 ma or 300ma or whatever.
  2. the transient requirement, for instance what happens when you open the relay while the sound is still present. There will be a transient voltage generated that could possibly damage a reed switch.
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