Maybe reverb is turned on. :^) Most sound drivers include a mixer and dsp settings for effects. Check that and make sure you are setup for 2ch sound. Make sure your speaker and headphone input are from the Front L/R connector on audio outs on mother board or card. Connecting to Rear L/R may cause the software to simulate surround sound and you are hearing generated rear channel audio from 2 ch input. Just some thoughts since you didn't get into too much detail on config.
