Error running AudioIntervals on ExampleData

I get a strange error when running:

a = ExampleData[{"Audio", "MaleVoice"}, "Audio"]
voiced = AudioIntervals[a, #VoiceActivity == 1 &, .1, 
  PartitionGranularity -> {.06, .01}]

Function::slot: Slot[#VoiceActivity==1&] (in
Slot[#VoiceActivity==1&]==1&) should contain a non-negative integer or
string.

Function::slot: Slot[#1==1&] (in Slot[#1==1&]==1&) should contain a
non-negative integer or string.

AudioIntervals::prop: #VoiceActivity==1& is not a valid test.

Could this be a Mathematica bug or some internal error? Normally the examples work well.

Any ideas, what the problem could be?

my Mathematica version is 12.1.1.0