I think casinos can be used as a good model for positively reinforcing a sense of achievement to the user. Specifically slot machines, as they make all types of pinging noises even when you are really losing money. This gives the user a false sense of achievement in a lot of cases but it can still be used as a good reference point.
This video summarizes a study around how slot machines use sound.
How Slot Machine Sounds Trick You into Playing More
I think high pitched sounds definitely give the user a sense of victory or achievement. This should give you a good starting point.
You could also, search for information on the psychology of slot machines or sound in general.
Also, some sort of counting system that keeps track of tasks completed could give a sense of achievement (like reputation on stackexchange). Maybe weight tasks based on expected difficulty.