Pause recording by selecting the pause icon. Leave the Transcribe pane open while recording. Start talking or begin a conversation with another person. Wait for the pause icon to be outlined in blue and the timestamp to start incrementing to let you know that recording has begun. That way, the recording can pick up the sound coming out of your device. If you want to record and transcribe a virtual call, don't use your headset.
For example, if your computer's microphone input is set to your headset mic based on the last time you used it, it won't work well for picking up an in-person meeting. Be careful to set the correct microphone input on your device, otherwise results may be disappointing.