can I output video to monitor, via HDMI, and audio to analogue speaker system, via 3.5mm connector?
Yes, you can. The “A1 OUT / A2 IN” 3.5mm connector…

… has two functionalities:
- it can output analogue audio that comes directly from the SCART input connector (pass-though mode)
- and it can be used as audio input connector in conjunction with the “V2 IN” Component input connector
There’s a toggle switch between the 3.5mm connector and the HDMI connector, to change the mode according to your needs.
will audio output on both devices, namely TV and speaker system, at the same time?
Since it is intended for the usage with gaming devices, the OSSC is designed for a low as possible latency between analogue input and digital output. But every monitor/TV will add its own latency to that – some more, some less. In the best case there’s no noticeable delay between the digital video output and the analogue audio output.