How much would it cost to soundproof a closet for music practice (at least 80 decibles not heard outside)? The closet is approximately 4x4x9. I am looking for approximate numbers or any suggestions on what I should research.

First you need to determine what the sound level is outside when playing. A scale decibel or true decibel?
Then you need to find something with an STL that will reduce the sound to a lower level. For example, if you meadure 80 DbA and want 50, you need something with an STL of 30 or more. Regardless of how you do the walls and ceiling, the door may be the week link because of the door openings. 5/8 drywall would be a good start. Isn't a 4X4 somewhat confining?

Vermiculite was used in the 90's in sound studios. Fill walls and ceiling. Use 5/8'' sheetrock.

When buying, be sure it does not contain asbestos. This would be a server health hazard.

Chuck

It is for singing. I need to be able to do my vocal exercises without annoying anyone. So 4x4 isn't too bad. Not sure about the scale/true, but a cheap decible meter reads 120 when I sing (opera), and I want it to block at least a reasonable amount.

I've gotten some advice about getting walls/ceiling filled with Rockwool, but can't find out how much that affects the sound. Have also considered carpet on floor, hanging fabric on walls, or putting up egg cartons to line them.