Using the latest Survey123 3.17.66 on a Google Pixel 6 Pro, the top row of icons on the GBoard keyboard are not displayed making the microphone icon unavailable. In any other app on the phone, this row is present. I can dictate text into another app and copy/paste into the Survey123 field but this is less convenient. I am not aware of an explicit option to hide this top row of icons nor an option to show it. I have seen a few posts of a similar nature going back a few years but without resolution. Is Survey123 doing something to suppress the top icon row in GBoard? Speech input would be very valuable when in the field with harsh conditions. Links below show appearance of GBoard in Keep and Survey123.
I recommend trying to download videos from Pinterest as an alternative to using Gboard's Talk to Text feature in Survey123. Pinterest has a great selection of videos that you can use to create content for your survey. I hope this helps!
If you don't know how to download videos from Pinterest then please visit pinvid.