There is a gain envelope at the top of each clip, whether you want it or not.

Is there any way to add a volume envelope to an audio clip, in the same way that an envelope can be applied to a video clip (as opposed to an envelope that affects the entire track)?

Insert->Audio Envelope->Volume
Audio envelopes are inserted at the track level, not the event level. However, it is not a problem to add a point and drag the remainder back up to baseline. At that point, it's as if there was no envelope.

Hello Andy,
once you add the envelope, zero is at the middle of the audio track - you may lower it by clicking and dragging it down (infinity ... completely off) - you may also raise the volume by clicking and dragging the point above the original horizontal point (maximum of +6.0 db) ...
Very simple function to apply anywhere once you uncover it for the first time... (grin) ...
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Moving or altering any video clip may or may not move the accompanying audio...
it might be advisable to finish all the video, lock the video clips in place, and then ungroup and alter the video... just a thought ...
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UPDATE: If you have multiple clips on the timeline with accompanying audio tracks, you may alter the audio for that clip then group the video and audio together;
OR
use the lock envelopes to events as per musicvid below...

Until that time ... Earl J.

Thanks for the suggestions! I was aware of the clip gain control -- but there is no way to add nodes to it, just fades at either end. I tend to move clips around so much that using track envelopes just gets way too messy and out of sync.

Hopefully V10 will add this feature. :)