@ReadyPlayaWon I'll keep that in mind in future situations, but I've had a chance to confirm and the video volume isn't an issue when in HDMI mode. The issue is only with the headphone jack. I'll continue looking for a programming solution.
I'd love to be able to use the attractmode with OSMC, but I think it isn't possible
I use OSMC for KODI and installed RetroPie via RetroSMC, which now works perfectly with Debian Stretch now that RetroPie has been worked on for Stretch
I've even tried custom scripts I found that add the attract options to emulationstation so you can switch between the different modes via the emulationstation menu but that doesn't work either as the modes don't appear on the emulationstation menu. At least not with OSMC as no menu options appear and I've never seen the 'Configure Splashscreen' option either with OSMC