Unless you're going to maintain two versions, I'd suggest leaving the QR image at whatever minimum size works for both print and screen.

Do note that, if you end up publishing in PDF format (which presumably won't have the clickable audio), whatever works on screen for you might not work on a screen with a different resolution or if the user views the document with a different scale. In that case, you might also want to publish some guidelines for viewing the PDF so that the QR image works as intended for that format.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]