This bug report is based on recent tests of Mojave developer betas running under Parallels VM hosted by High Sierra. I have not attempted to reproduce the problem on a native installation of Mojave. In Safari Technology Preview 61 - Version 12.0 (14606.1.30) - and some earlier previews, the audio element fails to play streaming MP3 audio produced from an Icecast streaming server. To reproduce the problem, visit the Icecast directory for MP3 programs - http://dir.xiph.org/by_format/MP3 - then choose any MP3 streaming program in the list and click the "M3U" link to start playing audio. The browser will attempt to load a standalone audio player page, but the playback fails with a message on-screen: "Unsupported Plug-in". This problem seems to affect recent daily builds of WebKit too. It also affects Mac apps that use an embedded WebView in Mojave. The MP3 playback for the test above works correctly in Safari 11.1.2 and earlier versions. I suspect a factor could be non-secure transport using http scheme instead of https. I have an open source app that could be affected by this problem - https://github.com/dsward2/LocalRadio
It plays in STP 62 on High Sierra, so seems like a Mojave regression.
<rdar://problem/42936101>
It also works in STP 58 and 62 on Mojave.
Looks like we cannot reproduce in any configuration. Douglas, perhaps you do have an unsupported plug-in that claims this type? Can you please check which plug-ins you have, and try disabling them?
Alexey - I'm checking the various Plug-Ins folders, but this is basically a pristine installation of Mojave beta in Parallels VM on a 2013 Mac Pro, so it should be the default set of plug-ins (if any). According to STP 61 Preferences, No Extensions are installed. The Web Inspector Console does have this error message with the "Antenne Bayern" station on the Icecast directory: "Failed to load resource: Plug-in handled load - http://dir.xiph.org/listen/223/listen.m3u" But it does not say which plug-in. There is also a warning message: "Tried to use an unsupported plug-in", apparently from HTMLPlugInElement.cpp. All of the other Icecast mp3 streams produce a similar result. I tried to reproduce the problem on STP 62, but it crashes immediately upon launch. Thanks for checking. I'll try again when STP 63 is released and report back.
I'm now thinking this is more of a Parallels issue. I tried Safari in macOS 10.13.4 under Parallels, and it cannot play the Icecast mp3 streams either. But the error message in the web view is different: "Missing Plug-in". Clicking on the arrow drops an alert box: "This webpage has content that requires an Internet plug-in. You do not have the plug-in required to view this content."
Thank you for the report! This does not appear to be a WebKit issue, so Apple sill continue to track it internally as <rdar://problem/42936101>.
I have reported this problem to Parallels support, and will update this page with their findings.