If you don’t do anything to the audio you add to a project, rendering to the project format won’t have any effect on it.
Rendering to a different format can potentially change it but that’s a codec issue, not a Vegas issue.
If you’re talking about the way it sounds on a DVD after encoding, that’s a Dolby encoding issue.
Assuming it’s an AC-3 render, you can now choose Dolby Digital AC-3 Studio (*.ac3) as the “Save as type” and Stereo DVD, AGC on as the “Template”.
In the days before Pro 8 and this option, the solution was to modify the standard AC-3 template as follows:
Encode set to AC3;
Click on Custom tab;
On the first tab, set diag. norm to “-31”;
On the last tab marked Preprocessing, set the Line Mode Profile and RF Mode Profile mode to “none”;
Save this as a preset.