Hi, because the background murmur of a lecture theater is a wideband noise source there isn't a neat way to do it. Selecting a period of noise to eliminate using the tools would help if the noise were not a random source (but it is).
Use the remove hum or hiss tool (left side of window) to apply a noise gate. It isn't an ideal application of the tool and it is likely to impact on the lecture you are listening to but is is an effective way of removing wide spectrum noise.
