What you are asking for is listen mode. It will give you the possibility to lower the music mix and rise the global level at the same time effectively rising the vocal level. But it will not work i think with direct monitoring. This is your problem.
Could you disable direct monitoring ? At least to try ?
Because your are using zero latency direct monitoring, Nuendo communicate to the Fireface to give her the mix level from channel 7 to the mix. But this does not take into account hardware implementation differences, specially the fader headroom available in the fireface mixer.
Another thing that is not taken into account with zero latency direct monitoring is the track effects that you could have inside Nuendo. If you have a compressor inside Nuendo, it will not be replicated during recording because Nuendo is bypassed on the recorded channel during input channel zero latency monitoring switching.
You’ll have to duplicate this compressor manually inside totalmix if possible, if your interface do have effect processing available at this place.
If the effects you have inside Nuendo on the recorded channels are not duplicated inside the zero latency monitoring path in your interface, then you’ll have EQ and or level differences when swithing to recording mode.
This is where it is interesting to have an interface more integrated with Nuendo at the monitoring level. I think that high end Steinberg and Yamaha Nuage interfaces do allow to setup effects in the ZLM path directly from Nuendo. You’ll have to ask to other users or read those interface manuals to know exactly what they can do here. I never tried them myself.
I think that another advantage to use advanced interfaces is that you can put exactly the same effects and EQ in Nuendo and in the Interface direct monitoring path because interface effect plugins duplicating those from Nuendo are available.
This is not possible with third party interfaces, for example you will never have the same compressor or EQ.
Another solution is to use an analog console for recording. Here you’ll have all the routing and monitoring perfectly working and ergonomically user friendly. And true zero latency monitoring too ! This is still today frequent inside studios where high end analog recording console are still available.
Your other solution is a dedicated mix for recording or grouping music channels so that you can lower music when recording voice.
I would be interested to know your results with listen mode, i’m curious to know if it is duplicated inside the interface zero latency monitoring path.