Researchers at Stanford University (USA) and the University of Edinburgh (UK) have tested various automatic speech recognition (ASR) systems and found that in general they have a more difficult time recognizing speech from males than females. One of the causes mentioned by the researchers was that the men tended to use fillers such as "emm" more often.
I should point out that the tests were conducted using recordings of telephone calls. This is important because the type of "channel" the audio is being carried over makes a significant difference to a computer-based speech recognition system.
The work was sponsored by the US Office of Naval Research.
For a quick overview of the research, you can read the BBC (center-left news media, UK) article here.