Microphone volume lowers upon exiting 'Sound' window

Hello, I have an Acer Aspire E5-572G with Win8.1, and SpeedLink Thebe CS headset.

Since the laptop has only one jack for both audio output and microphone, the headset is connected through an adapter (2x 3.5mm (audio/mic) to single 3.5mm).

A problem occured when i got on Skype. Other people could hardly hear me talking. I went to Sound > Recording > Microphone Properties to see what I can do about it. Soon, we figured out that they could hear me just fine while i was in the settings.

More precisely, it seems that upon entering the Microphone Properties window my volume becomes normal - without even changing any settings - and upon closing the Sound window it changes back to ridiculously low. (Basically, I had to have the Sound window opened in the background for the duration of our call.)

I tried meddling with various settings in both Microphone Properties and Speaker Properties, and of course Skype itself, but nothing solved it yet. Interestigly enough, though, in Skype's microphone settings there is that line with a green bar indicating my volume, and it reflects what i said above: when i have Sound window opened, the green bar easily fills out the line upon any louder sounds. Otherwise, when i don't have the Sound window in the background, it hardly fills a fifth of the line. That makes me think the problem is not in Skype settings themselves.

Any idea on what could cause such behavior?

EDIT: I was testing some more via Skype's Echo service and found out that, actually, the volume is normal while the 'Recording' tab in 'Sound' window is active, otherwise it is low.