Thank you for reaching out to the Microsoft Q&A forum.
I understand that you are experiencing an issue where the speech recognizer sometimes detects random words as input even when there is complete silence. This issue could be caused by the microphone sensitivity being too high, causing the recognizer to pick up background noise and interpret it as speech.
To mitigate this problem, you can try adjusting the microphone sensitivity to a lower value. You can also try using noise suppression techniques to improve the speech recognizer's performance in noisy environments. However, please note that the Azure Speech SDK does not provide an option for noise cancellation as it uses the audio from the selected source (file or microphone) as is and passes it to the service.
A workaround could be to use an external microphone instead of the device microphone which can pick up other sounds or use the speech devices SDK which is actually a hardware device with the speech devices SDK and has the capability of noise suppression and echo cancellation.
If you have already tried adjusting the settings and using noise suppression, and the issue still persists, you can try some of the other solutions mentioned in the Azure documentation on how to mitigate or resolve echo issues.
I hope this information helps.