The Dlink does offer a RTSP template so try this. I have the DCS-2332 (similar for outdoor) which i use with audio 24/7.
For the Foscam, if over Onvif you don’t get audio then probably there is no way to get audio since by default it should already redirect you to its RTSP stream.
As for the Amcrest, maybe @Henrik can investigate further if it’s possible to connect them over RTSP.
According to Amcrest both cameras are ONVIF. But that do not work?
There is already a template for Amcrest IP3M-943 using rtsp for high res. video. That camera do not have audio and no PTZ.
Amcrest IP2M 841 is with PTZ and audio and therefore it would be great if it works with onvif. If not, use the same template as earlier to get the rtsp stream which now should include video and audio, but not PTZ.