I have a little history with Prof. Fischer-Dieskau. I have sung in chorus many times when he was the soloist, attended rehearsals with him, masterclasses he led, heard him in recital on several occasions, and once turned pages for the continuo organ during a concert, where I was no more than 6 feet away from him. During these experiences I heard all sorts of variants (non-Siebs or Duden standard) in his pronunciation, curiously regarding consonants, not vowels, and not just Berliner or Wiener dialect.

In the case you cite from Die Schöne Müllerin, I know many times in his masterclasses he stresses the importance of "role" and "character," and would show how one can use dialect, for example, to enhance the overall presentation. All of his non-hoch-Deutsch variants that I heard, once I contemplated the alternatives, proved to be a very musical pronunciation.

The hard "k" for "Sag, Bächlein," although appropriate as hoch-Deutsch, probably was too percussive for his musical taste, therefore he softened it for musical reasons, as well as dramatic/dialectal usage. There is a tradition for this type of thing among German Lieder singers, and you would be hard pressed to find any German artist who does the hard "k" in that phrase, even though it is "more correct," phonetically speaking.

So, I would encourage you to also take the musical approach, relying on your imagination and how you want the passage in question to sound...with softer or with harder consonant, and to teach your students likewise. It's ultimately about the music, the rules are subservient to the cause.

There is a rule to be observed, the so-called "Auslautsverhärtung" which you can look up in ChoralNet, as it has been discussed here before. That rule does not necessarily apply to every situation you are citing (poetic shortening of a word), but does apply in the case of "Tag," where you correctly pronounce "g" as a "k." However, once the rules have been taken into account, if the music can be enhanced by slight departures...well, I guess you get my opinion.

There are of course regional differences, and perhaps your two German expatriate friends are from different parts of the country. One really can't say, "The Germans sing it this way..." as not all Germans sing all things exactly the same. Most things, yes....

Rick