In the meantime I have the right cable and was able to connect the monitor.
But now the sound no longer works. The little I have found via googling has not helped.
If I take the cable from the second monitor off the laptop and restart the laptop, the sound works again.

How can I get the sound to work even if the second monitor is connected?

Many thanks for help :)

I mean, you need a cable to connect the monitor to your laptop anyway. If you have an HDMI port on your Lenovo Yoga 730-13, an HMDI to HMDI cable will do, otherwise you'll have to look for alternatives like USB to HMDI.
Hope this helps!