I watched a video last night produced by Adobe to help contributors learn more about submitting stock images and footage. One of the comments made was that video should have no sound. There was nothing said about why though. I can see why if the video is shot in a noisy environment but what about videos where the sound is an important part of the story? For example a songbird singing in a tree? Or a rushing waterfall like the one in the link below? Wouldn't the audio be an important part of the clip then? Could buyers not easily mute the audio if they choose? https://stock.adobe.com/stock-photo/id/231814713
If the ambient sound is high quality and relevant to the clip then it's OK to include. If however there is any unnecessary audio or if the audio lacks professional quality it will likely result in a rejection. The example you noted of a chirping bird is fine though it's likely the end user will remove the audio for their final use as most stock clips are being added to larger projects.
If there are any voices in the audio you will need to supply a model release for each voice regardless of whether you see the person speaking or not. As a general best practice I recommend you do not include audio. Exceptions can be made however so use your best judgement.
I hope the video you watched was informative and entertaining
-Mat Hayward n
Thank you and yes it was.