is there one in audacity?
if not, is there a way to achieve the same effect?
There is a “Time Track” feature that may do what you want: http://manual.audacityteam.org/o/man/time_tracks.html
thanks!
it did what i wanted, but i was wondering if i could make the time track affect only one track, not everything in my mix.
Yes you can, sort of 
You would need to create the time track envelope (this will affect all tracks), then:
Select the track that you want it to affect.
Tracks menu > Mix and Render. This will “render” the selected track and in so doing will “apply” the time envelope to the track. The selected track(s) are “used up” (disappear) and a new “Mix” track is created at the bottom (below all other non-selected tracks).
Then delete the Time Track (click on the [X] in the top left corner of the track.
See here about “Mix and Render”: http://manual.audacityteam.org/o/man/tracks_menu.html#mix