@wordpress/block-library/src/audio/style.scss

.wp-block-audio {
	// Supply caption styles to audio blocks, even if the theme hasn't opted in.
	// Reason being: the new markup, <figcaptions>, are not likely to be styled in the majority of existing themes,
	// so we supply the styles so as to not appear broken or unstyled in those themes.
	figcaption {
		@include caption-style();
	}

	// Show full-width when not aligned.
	audio {
		width: 100%;

		// The browser natively applies a 300px width to the audio block.
		// We restore this as a min-width instead, for alignments.
		min-width: 300px;
	}
}