Mysterious switch on side of Data Recorder

[querjek]

I recently acquired a Data Recorder that has an extra switch installed--see pic, top unit. Any idea what this is? I haven't had a chance to open it up to see what's going on yet.

Update: it's connected to the speaker? I don't have any audio tapes here to test in it, unfortunately.

Update 2: I found a tape! The switch turns the speaker off and on. No clue why this was added since the volume slider works fine.

[P]

Maybe it's a homemade mod? Kinda looks homemade to me.

I'd guess it's for not having to listen to the data in audio form while loading. If turning down the output volume dial the Famicom can't hear the signal either and loading won't work.