Google released an update for the iOS search app that resolves the issue with a muffled and low-volume voice prompt. Version 14.1 of the app says "fixed a bug causing low audio volume for some users."
There were several complaints about this in the Google Web Search Help forums over the past couple weeks.
One user described it as a "muffled" sound coming out of the app, making it hard to understand the voice response from the app.
AJ from Google responded Thursday saying:
Thanks everyone for your reports here! Sharing the good news that we've rolled out a fix in a dot release (14.1), which you should start seeing it in the App Store shortly. If you don't have auto-update on, please manually update your Google app version for the fix.
If you continue to experience any issues with low volume on the iOS Google app after updating to 14.1, please report back on this thread for us to investigate. Thanks!
Forum discussion at Google Web Search Help.
Note: This post was scheduled to be published today and was written on April 28th.