George Welch was the answer I was looking for. His 1 October flight was the first time that a sonic boom was witnessed by others. Mutke may well have been first, but not only is it unconfirmed, there are also reasons to doubt it. At the end of WWII, german production quality was not good due to shortage of materials, depleted workforce, and constant air bombardment of the factories. The 262 was not built as strongly as American aircraft were of the time. So, there is the question of whether a 262 would have been able to withstand both the buffeting approaching Mach 1 and the repeated buffeting slowing down again.
Welch's flight was kept quiet for a few reasons--at the time it was determined that a military pilot would be given the honor of making the first supersonic flight, and Welch was a civilain test pilot at that time. Also, the government did not want to announce that the XP-86 went supersonic because it would nullify the huge expense of the X-1 program. They wanted to have something to show the public for the money spent. So, Yeager went down in history. it was later released that Welch went supersonic in the XP-86, but they claimed he made the flight months after he actually did.