Bug fix, change volume to StringProperty
Bug fix, change volume to StringProperty

cmusv-sc/DIWD-Team4-Wei-Lin-Tsai,cmusv-sc/DIWD-Team4-Wei-Lin-Tsai,cmusv-sc/DIWD-Team4-Wei-Lin-Tsai,cmusv-sc/DIWD-Team4-Wei-Lin-Tsai