@BMitch has the right idea with furnishings and fabrics - it's the simplest, fastest and cheapest.
Also, you can add soundproofing in other ways. Have a look at
- this question on soundproofing a large wallthis question on soundproofing a large wall
- this one on ceilingsthis one on ceilings
- and this onethis one which has some good discussion on acoustic damping materials.