Hi, Brebda. Sound travels through hard things and is absorbed by soft things. Wallboard is pretty hard so the sound goes through your outside wood walls into the studs and on through to the wallboard. Putting something softer in between will help but sound also travels through window glass so things like your window arrangement will hav a big effect on this.
If you want to try the homasote, you will have to remove the wallboard, install the Homasote and install new wallboard over it. Then you will have to mud and tape the joints, prime and paint. This will be a pretty big and messy job. I would not even consider taking out plaster walls for this. But before you start this, check out Homasote's web site at www.homasote.com. They also make a product called Soundasote (I believe) for this and it is supposed to be better. It also has limited availability so check with the Homasote people to see what they think about this and tell them where you live.