I had similar situations this week in #1024, in two different ways: - The ffmpeg can't input mpeg (this include ffplay), but lib-vpx is normal. The Xenocara does not freeze, it just can't play; Reproduced in i386 #1024, almost all mp4 files this happen, but may be just a upgrade bug. - The 'mupdf' could not open a specific file. This one freezed all Xenocara. I came to root, and then killed the process manually. Can't reproduce.
I'll try with a amd64 snapshot, if possible, soon. dmesg: http://marc.info/?l=openbsd-misc&m=143692017331643&w=2