The Audio Search options enable you to search for words and phrases spoken in the call or spoken in given sections of the call - for example words spoken only within the first 30 seconds of the call starting.
You can also search for calls that either don't contain those words or phrases at all, or don't have them in given sections of the call.

You can select the 'minimum confidence level' for the search in the options. The confidence level is the probability that the call contains the phrase searched for, so you can say you only want to see calls that have over 70% chance of having that search term.