Yep. Under the sound object, you can set the volume of a sample to any number between 0 and 100. So you can always set the sample volume to its current volume plus or minus a bit.
Also, there are several different "Sound Channels" you can use, and adjust their volume levels independently. I think the proper Event is "Play Sample on a Specific Channel" or something like that.
Select it to play background music for example, and then pressing certain keys or clicking on certain things on-screen can be linked to adjusting just that Channel instead of everything at once.