On RedHat 7.1 using a Mozart ISA sound card in an Intel P-75 system, the "mem free" as observed in 'top' steadily decreases while playing any mp3 in the default mpg123. The memory leak stops when the player stops, but the "mem free" does not increase or return to a previous state. The memory leak still occurs if I change the sound card to a PCI ESS Audiodrive. This does not occur on RedHat 7.0 using a SoundBlaster 16 ISA sound card in an AMD K6-2/500 system (playing the same mp3 samples, of course).
Installed RedHat 7.0 on the hardware, and the same problem occurs.
It's being used for cache. It's completely normal; the cache will be freed if the memory is needed for something else.