OGS changed their CDN URL so that info doesn’t work anymore, but the good news is that it’s ever so slightly easier to do now.
This is for Chrome. First, get the Requestly extension.
Once you open the Requestly dashboard, click the plus icon on top. From there create a new Cancel Request rule, select matches from the dropdown list and copy paste the following into the source URL field:
/cdn\.online-go\.com\/.+female-en-(10|9|8|7|6|5|4|3|2|1)\.[a-z]+/
This prevents all countdown audio. If you want to keep any of the specific numbers, simply remove those numbers from the list (and keep only one bar between the remaining ones). So, if you want to hear 3, 2, 1 you would have:
/cdn\.online-go\.com\/.+female-en-(10|9|8|7|6|5|4)\.[a-z]+/