If you've got a B7 then you don't have MMI you have an RNS-E (which says MMI on the startup screen because of an idiotic decision by a mentally deranged Audi marketing person that has led to hundreds of B7 owners buying an MMI sat nav disc which won't work and means they've bought a £200+ coaster).
Do you mean adjust the volume of the connect/disconnect tone, or the volume during a phone call?
To permanently adjust the volume during a phone call, just adjust the volume during a phone call - the setting will be remembered.
There isn't a guaranteed answer to permanently adjusting the volume of the connect/disconnect tone - for some people (me included) this works: go into Engineering Mode on the RNS-E and select the screen that shows the startup volume and set that to a lower number. BUT, this doesn't work for everyone, and there doesn't seem to be any common factor in who it works for and who it doesn't work for...