This is from the user's guide:
By default, MediaLab will record the voice response until the subject clicks Continue. You can optionally specify the duration parameter to record the voice response for a fixed time. E.g., (d5) would record a voice response for 5 seconds and then automatically continue to the next item.
Since you can use the spacebar instead of clicking Continue, would that work for you? i.e., using the spacebar as your input key?