well slapshot, it depends. If u used my engine sound, u probably will not be able to increase it much more on the account that I have them alrdy peaked to -1 to 0 DB. Any more increase and it will start to distort or the sound card won't play it any louder. Other problem could be u are not doing it right (no offence intended). I use Adobe Auditions pro 3.0 do create my sounds. In there its very easy to lower or raise the volume. As far as the windows recorder, I have never used it so I can't tell yea if it would work or not.
To be honest, I would wait to modify any sounds till HTC gets things worked out. If you do decide to modify sounds, I strongly suggest make a backup of the original sounds first.