Sound is broken on http://websdr.sp3pgx.uz.zgora.pl:8901/

We have a report in our blog about broken sound on http://websdr.sp3pgx.uz.zgora.pl:8901/ (https://blog.torproject.org/tor-browser-706-released?page=1#comment-271626). The website claims that "Your browser does not seem to support HTML5 WebAudio", yet it worked in Tor Browser 6.x and it works on Firefox 52.4.0esr.