These live performances were recorded for broadcast during WWII in Germany, and while the sound is not up to modern standards it is surprisingly good for its time. The microphones in the concert hall were wired to a small, windowless control room, where they were primatively "mixed" and the signal sent via telegraph wire to the radio transmitter studio, where it was recorded on early Magnetophone tape recorders. The tapes were captured by the Soviets after the liberation of Berlin and transported to Moscow, where they languished for many years. Some performances were released by the Soviets, but the tapes were eventually returned to Germany and reprocessed in the 1980's.
The microphones used were omni-directional and surprisingly sensitive, and while there is some compression of dynamics, there is a surprisingly good sense of hall spaciousness and resonance for the mono source -- along with the inevitable coughs, rustles, etc. And the sound quality varies, depending on the quality of the tape (some were recorded over several times, and the tapes themselves may have suffered some damage during their years of storage) and the alacrity of the "engineers" in the windowless "control room" in the old Philharmonie, who had to adjust volume both to capture soft passages and avoid overload in the louder ones. As a general rule, the performances with soloists suffer the greatest from congestion and distortion in the climaxes. However, the sound of the Berlin Philharmonic during the war years comes through surprisingly well. DGG has favored clarity above all, and has not filtered out distortion in the string sound, etc. In general, the more "analytical" your sound system is, the worse these recordings will sound. Those who, like me, prefer a more analog-oriented sound will fare better.
The performances themselves are notable for their intensity of expression. Both conductor and orchestra seemed to be playing as though their lives depended on it (as indeed they did, considering how ofter the Nazis threatened to disband the Philharmonic because of Furtwangler's refusal to kowtow, and send all the musicians into the army). Furtwangler, living with the ambiguity of his decision to stay in Germany and minister to the spirit of the German people through music, and the inevitable compromises he made so he could continue his mission, conducted with a controlled fervor rarely matched.
In this volume, the highlights are the Beethoven pieces: a mystical Beethoven 4th, an heroic and fierce Beethoven 5th, an almost maniacal Beethoven 7th with an intensely dramatic second movement and a finale nearly spinning out of control, and a lovely violin concerto featuring the Philharmonic's concertmaster as soloist. There is also a magnificent and dramatic Schubert 9th. All of these should be required listening, even if you already own Furtwangler's post-war recordings, because of the difference in interpretation. Post-war Furtwangler is more spiritual and detached; mid-war Furtwangler is more emotional and in the thick of things. Each has its virtues, but don't assume you know everything about Furtwangler -- or about these pieces of music -- until you've heard both.
Highly recommended despite the variable and limited sound.