Because you're going to get into major distortion if you try to go too slow or too fast the range is limited. Right click on the audio track, properties, then time stretch. Try going up and down a couple points under change pitch preserve length, semitones for music.
Generally, there are two ways people want change audio speed: They either want the pitch to change, like changing speeds on an old record player, or they want the pitch to stay the same, but have the audio play faster or slower. Vegas provides a method for each.
From the Vegas Help file:
Right-click the event and choose Properties from the shortcut menu.