Unless there is a setting somewhere, I doubt there is a straightforward way.
The speaker which emits the sound appears to be built into the top of the instrument cluster:
If, after removing the instrument cluster, the two speaker wires are exposed you could theoretically attenuate the signal to the speaker with a couple of resistors and a soldering iron. This would of course make all audible warnings quieter.
According to topix, the speaker can't be replaced separately so if you damage it whilst fiddling you'll be ordering a new instrument cluster (not cheap) and trying to come up with a good cover story.