After reading https://stackoverflow.com/questions/...o-output-sound it definitely sounded like the problem was that ffplay needed to be patched for Sailfish to use libaudioresource, so I decided to give it a go, and depsite not having a clue what I was doing, it seems to work!

Attached is ffplay.c and ffbuild/config.mak. There may be some devel files that need to be pkcon install'ed, but I can't remember for sure, but it should be obvious from any compilation errors if there are.

The screen does go to sleep during playback, but https://openrepos.net/content/xmlich02/stay-awake can be used to work around this.

EDIT: In case anyone wants to try this, these are the devel files that I currently have installed, though I don't think all of them are required:
Installed SDL2-devel-2.0.3-1.4.1.armv7hl
Installed SDL2_sound-devel-1.0.1-1.2.3.armv7hl
Installed glib2-devel-2.48.0+git3-1.11.2.armv7hl
Installed glibc-devel-2.19+6.9-1.8.1.armv7hl
Installed jolla-developer-mode-0.3.26-10.27.4.jolla.armv7hl
Installed jolla-developer-mode-profile-0.3.26-10.27.4.jolla.armv7hl
Installed libaudioresource-devel-1.0.5-1.4.3.armv7hl
Installed libxml2-devel-2.9.1-1.3.5.armv7hl
Installed pcre-devel-8.31-1.3.1.armv7hl
Installed perl-devel-2:5.16.1-1.1.29.armv7hl
Installed pulseaudio-devel-8.0+git5-1.21.2.armv7hl
Installed smpeg-devel-2.0.0-1.1.6.armv7hl
Installed zlib-devel-1.2.8+git2-1.3.21.armv7hl

And to resize and rotate (for my device) I use:
ffplay -i <filename> -vf "transpose=1" -x 540 -y 960