I was debugging a problem with my own snap using webkitgtk not playing audio at http://kolnogorov.me/webpreview/samples/test-article-audio.htm. During debugging, I've tried epiphany snap and it's unable to play it as well:

With GST_DEBUG=2 I found that webkitgtk/gstreamer want to create a temp file in /var/tmp:
0:00:00.234583658 5205 0x5f7e0b829ec0 ERROR webkitcommon GStreamerCommon.cpp:927:operator():<media-player-1> Got message: error message: 0x7ad564007de0, time 99:99:99.999999999, seq-num 350, element 'downloadbuffer2', GstMessageError, gerror=(GError)NULL, debug=(string)"../plugins/elements/gstdownloadbuffer.c\(940\):\ gst_download_buffer_open_temp_location_file\ \(\):\ /GstPlayBin:media-player-1/GstURIDecodeBin:uridecodebin1/GstDownloadBuffer:downloadbuffer2:\012system\ error:\ Permission\ denied";
0:00:00.234640600 5205 0x5f7e0b829ec0 ERROR webkitmediaplayer MediaPlayerPrivateGStreamer.cpp:2034:handleMessage:<media-player-1> Could not create temp file "/var/tmp/WebKit-Media-XXXXXX". (url=http://kolnogorov.me/webpreview/samples/audio3.mp3) (code=5)
The following layout has helped:
layout:
/var/tmp:
type: tmpfs
You may want to apply it to epiphany as well. Or maybe even apply it at gnome extension level to solve this for all applications using webkitgtk.
I was debugging a problem with my own snap using webkitgtk not playing audio at http://kolnogorov.me/webpreview/samples/test-article-audio.htm. During debugging, I've tried epiphany snap and it's unable to play it as well:
With GST_DEBUG=2 I found that webkitgtk/gstreamer want to create a temp file in /var/tmp:
The following layout has helped:
You may want to apply it to epiphany as well. Or maybe even apply it at gnome extension level to solve this for all applications using webkitgtk.