The 1.6 has two TRS jacks; one is for audio input for AV3, and the other is a combo audio breakout for AV1 and audio input for AV2 (togglable with a switch between that jack and the HDMI port).
I can think of a few approaches. If you want to fit a TRS jack to your OSSC to use that as an input in lieu of the input on the SCART jack, I suggest also adding a switch so you can toggle between the SCART audio and the TRS jack so you don’t unintentionally put too much voltage into the OSSC’s audio board or risk leaking voltage into another connected device.
Alternatively, you could pick up an automatic audio switcher, run your audio into that, and then either try to run that into the SCART input on the OSSC with an inline SCART breakout, or use that breakout to run your SCART audio into an audio switcher, bypassing the OSSC altogether.
Of course, if you have the budget, just get an OSSC 1.6 and either resell the 1.5 or use it to process video for your TOSLINK-capable consoles, if any.