We have built many a fine custom home utilizing fibreglass batts to reduce sound transmission through bathroom and bedroom walls. The result is excellent and Owens Corning now makes batts specifically for soundproofing, available at Home Depot or other home centers. While I agree w/ Henry that between most walls it is unnecessary, I have insulated every bathroom wall I have built for the past 10 years and have been extremely happy w/ the results, as have my customers who never thought to ask about it. Homosote is a good product, but will add thicknees to your walls and this may affect door jambs, etc. Good Luck.