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.