Cannot allocate memory while playing

I'm using LMS 7.9 1412783275, on Debian Wheezy.

After loading LMS takes 53.7% of memory on a 4GB machine - about 2.2GB.

The problem is when I try to play a file I get:
"Slim::Player::Song::open (593) Error: Cannot allocate memory: While creating conversion pipeline"

in the server log file.

Any pointers what should I check to see where the problem lies?