is your sound attached to a timeline?
if yes, and your movieclip is root, use
// if your movieclip is not root, replace root by your movieclip's instance name.
If you want to do it visually you can easily adjust the volume from the timeline from the properties of the sound layer. For example I dropped a random Metallica song on this layer named sound (you can see the waveform). Make sure the layer is extended out (click in the frame on the layer and press F5) so you can see the waveform. While you still have any frame in there selected take a look at your properties panel (Window->Properties if you don't have it open).
You have some simple sound options here. In the picture below you can see the sound layer, the waveform, the properties panel for that sound layer and if you look at the "Sound" section there is a setting named "Effect". You can choose an effect from there or if you want to handle your volume yourself easily, choose "Custom". That will open up a keyframe-based volume editor. Very easy to use. Just click on the box handles and move down to lower the volume or keep it at the top for max volume. If you want it to change over time, click the line anywhere to add a new keyframe.
Note you will see either 1 or more waveforms here depending if the sound is mono or stereo.
Change the volume how you like and press the play button to sample your changes: