Fix crash with more than one audio stream #716

Job	Run time
build-macos-brew-Qt6 (clang-9)	3m 8s
build-macos-brew (clang-9)	2m 42s
build-macos-no-brew (clang-9)	5m 19s
build-macos-no-brew-Qt6 (clang-9)	6m 21s
build-linux (gcc-9)	4m 53s
build-linux-Qt6 (gcc-9)	6m 2s
build-windows-mingw (5.15.*)	9m 57s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	11m 32s
build-windows-vs (5.15.*)	8m 56s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	13m 45s
	1h 12m 35s

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