Materials for building a sound proof home office in a 5 ft x 6 ft x 8 ft cube
How many dB of isolation are you looking for? The ideal would be to isolate the inside and outside from each other, which means offset studs and joists. The walls and floor could be filled with sand to achieve further isolation, if your building structure will handle the weight.... websearch home recording studios,  many if the solutions for those would apply here. 

Remember to allow for ventilation; any lighting or PCs in that box will be significant heat sources, and your own body puts out on the order of 100W of heat. Getting airflow without losing sound isolation may be tricky.

Easier solution: a good pair of noise-cancelling headphones may be all you really need.