Simple techniques:

I made a holder out of dowels and paint-stirrers to keep the dulcimer from being pressed into the softness of my lap and clothing. It helps.

Also, try using a thumbpick, which brings out the melody. I use plastic thumbpicks for this.

Keep the dulcimer truly in tune, using a tuner or tuning fork, as the whole instrument is designed to respond to certain frequencies. I learned this when I got my first guitar tuner and tuned my guitar exactly right. It was suddenly much louder than before.