According to their FAQ they actually want those poor conditions to be present in the corpus.
> We want the audio quality to reflect the audio quality a speech-to-text engine will see in the wild. Thus, we want variety. This teaches the speech-to-text engine to handle various situations—background talking, car noise, fan noise—without errors.
> We want the audio quality to reflect the audio quality a speech-to-text engine will see in the wild. Thus, we want variety. This teaches the speech-to-text engine to handle various situations—background talking, car noise, fan noise—without errors.
https://voice.mozilla.org/faq