To sound as if someone is turning up the volume in the middle of a song. (I’ve got ver. 2.1.2)
You could try the “Envelope Tool” (http://manual.audacityteam.org/man/envelope_tool.html) but note that if the starting volume is too high, increasing the volume will cause distortion. Ensure that your track has sufficient “headroom” to allow the volume to be increased without distorting.