Audio recording help.

I am trying to record sounds from my piano to my Pc, using audacity. The sound is ok for a little bit, but then the volume changes, and it never seems to capture the full sound from my piano, when I am playing it using headphones. I am connected using a 1/8–¼ adapter, do I need some sort of stabilizer to keep the sound steady?

Also, I need to be able to hear the sound from the piano as I am recording. Can I just re-route it to my headphone jack through audacity, or do I need to get some more adapters?