Bow make distinctive sounds when they are released, and one way to reduce that sound is to use muffles. Bow muffles are essentially furry or stringy balls attached near the ends of the bowstring, and significantly reduce the amount of noise made by the bowstrings. The trade-off is a significant reduction in power and sometimes accuracy. Most bow muffles are rather cheap, and home made versions work quite well.

Bow Muffle: costs 4gp, widely available wherever hunting or archery is common
Penalty: reduces ranges by 5 squares; -4 penalty To Hit
Bonus: when making a ranged attack by bow, the archer maintains concealment after firing (for both misses and hits)