Don_S
A couple of questions/suggestions.
What music server are you using?
Do you have the latest firmware on the router and music server? If not you may want to upgrade the firmware on each. I would start with the music server and see if it makes any difference.
Do you know if the music server has a jack for an external antenna?
Who is your internet service provider?
Not all wireless G or N servers have the same range. They all operate on the same frequency band but each design is unique. It's like FM tuners. They all operate on the same band but some are better than others.
You may want to read this. I think it well summarizes the stuff folks have mentioned in this thread.
http://www.microsoft.com/athome/setup/wirelesstips.aspx