in what pH level will yeast live?

First, in the environment, it requires the proper moisture level. The temperature of the dough is also very important for the yeast functionality. A range from 27° to 35° C is normally necessary. The yeast also requires the proper pH, or acidity level. It can function in a wide range of pH from 2.4 to 7.4 but in order to optimize its activity it is a good practice to maintain the pH of the dough in the range of 4.0 to 6.0.

http://www.gftc.ca/articles/2006/baker0809.cfm

Stomach acid is at a pH of 2.0 s.u. It digests the breads, yeasts, yogarts (which can contain live cultures), etc.

When making yeast rolls, the yeasts are added to a warm mixture of sugar and water. Water, for the most part, is neutral.

My swag then is around neutal to mildly acidic. I would say nothing below 5.0 pH, and to be a little safer, nothing below 6.0, unless you are running several test tubes to test it..

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Usually temperature is the main factor for a good level of fermentation.