I suggest just carrying a tape recorder with the sound of a shotgun being racked. Hit the "play" button as needed. 

Using a blank in self defense will get you killed. I shot an armed robber four times during a gun fight and had to use a blackjack before I could get him cuffed. He died in the hospital two days later but had plenty of fight in him after being shot w/my thirty-eight.
I agree with what everyone has said regarding this as a very bad idea.
Absurd.
Short Version: For your particular concerns, get a Judge or Governor and use .410 shotgun shells instead.
Long Version: I read thru a lot of these posts and frankly I'm of the "absurd" opinion as well. Since there are a lot of reasons that have already been stated, let's just say that I agree with about 99% of them and just move on to "other options"...
If you are searching for a less-than-lethal option because you are concerned about possible injuries to others when throwing lead, then I would suggest a pistol that would take .410 shotgun shells as an alternative. Here are my reasons:
1. They CAN be less-than-lethal;
2. Depending on the .410 round you choose you can still do quite a bit of non-lethal damage, where almost all of the choices will do more than a wax-filled blank. If you are pulling a weapon then your objective is to END the confrontation as quickly as possible and to heck with being polite;
3. A .410 shotgun round from a pistol can have very limited range and potential for ancillary injury to other persons. It drops off significantly with distance - much more so than any bullet;
4. You may bear less liability using a factory round in a self-defense situation as compared to using one you produce yourself;
5. And finally, if you want to load a bullet in your pistol - you certainly can, but at least you have the option.
I rely on high quality roman candles for self defense. Scares the hell out of them.
Just when I was considering posting about people continually 'over thinking' things, this appears!
Seriously..I am amazed. Singularly, very likely the dumbest post ever in a gun forum.
Using a blank in self defense will get you killed. I shot an armed robber four times during a gun fight and had to use a blackjack before I could get him cuffed. He died in the hospital two days later but had plenty of fight in him after being shot w/my thirty-eight.
Where did you hit him, and what kind of ammo were you using?
Where's Muss when we need him?