You mean, the Volume keyvalue?
10 is the default so if that isnt loud enough for you your other option is to stack two or more of them. That creates an issue with lag for people with lower-end computers though, so do so sparingly
If you dont want to do that, download Audacity. Its free and you can increase the volume of the actual file itself. but don't increase it too much- any sound that peaks over 20dB will distort badly.