That TRS jack to the right of the SCART input serves as an analogue audio breakout for AV1 OR an audio input for AV2, which is determined by a small switch next to it; it cannot function as an audio input for AV1. Generally, SCART carries both audio and video, so I think it’s a little unusual to need to separately connect audio when using SCART.
Additionally, I believe when you connect something to that TRS jack when it’s set to be an audio breakout for AV1, the OSSC will not also send that audio over HDMI; so, if your SCART cable already carries audio, and you’re connecting something else to that TRS jack, that could be why you’re not getting any audio.