Humax Box problems connecting with DVD player

I have a Humax freeview recorder set top box. It is connected via SCART to the back of the TV. The problem is that once I add my DVD player to the equation, it all goes wrong. I connect the Humax box to the DVD player via Scart as well. But the sound is problamatic. Firstly, if I am watching a DVD and the Humax starts to record something on the TV, we suddenly lose all sound on the DVD that is playing. Also, the sound on the DVD that we are watching is barely registering so you have to turn it right up. Third problem is that the Tv often does not register the Humax box - you have to turn the two off, then turn on the TV and leave for 30 seconds, then turn on the Humax box. Any suggestions for what is going on would be gratefully received! Thank you very much