How do I switch sound/mail notifications on or off in MSC?

You can turn message notifications off and on in the MSC profile of the user.

Click on the Settings icon in the upper right corner.
Within the Teams section, select the user you want to enable/disable the message notifications for.
In the user profile, scroll down to Message notifications.
Here, you have the possibility to enable or disable sound notifications and email notifications whenever a conversation comes in.