Foliage does not block sound. Japanese maples do tend to make dense summer foliage (good for screening at that time of the year), you might try 'Shishigashira' for one that has a suitable growth habit for your situation. All of the many varieties of Japanese maple need good drainage. A solid, intact pipe will not be affected by tree roots, these are only attracted to leaks. Even then a Japanese maple is not the type that would be particularly likely to clog pipes, if anything your existing hedge would probably get in their first (unless much farther away). The maple will not bother the hedge noticeably unless branching right against or over it, if you ever want to walk between the two you need to plant the maple far enough to the front to allow for that anyway.

Thanks for the help bboy! Very helpful infomation.