About MP3 / Audio URL QR Codes
Sound carries things text can't: the actual timbre of a voice, the phrasing of a spoken language, the specific recording of a song rather than just its title. An audio QR code links to a hosted sound file - an MP3, a podcast episode, a voice recording - so scanning it begins playback directly on the listener's own device, sparing them the trouble of searching for a file by name or guessing which platform hosts it. As with the video and image types on this site, the code itself contains nothing but the file's web address; the audio has to already live somewhere publicly reachable before the code can point at it.
Its natural home is anywhere hearing something matters more than reading a description of it. A greeting card can link to an actual recorded voice message rather than a printed sentiment, which changes the entire emotional register of the card. A museum audio guide stop can link to a curator's own commentary on a specific piece, delivered in their voice rather than paraphrased on a placard. A language-learning flashcard can link to correct native pronunciation, something no amount of phonetic spelling fully captures. A memorial marker can play a recorded eulogy, a voicemail kept from years earlier, or a person's favorite song, giving a visitor something a granite inscription alone cannot. A small independent record label might print a code on vinyl packaging linking to a bonus track that never made the pressing, rewarding someone who bought the physical record with something digital-only listeners don't get.
One thing worth testing before you commit to this type: not every phone begins streaming a linked audio file the instant the page loads. Depending on the file host, the visitor's browser, and its settings around autoplay, some phones start playback immediately inside the browser, while others prompt a download instead, leaving the listener to go find the file in their downloads folder and open it manually - a noticeably worse experience than the instant playback you're likely picturing. This behavior comes from the hosting platform and the visitor's own browser settings, not from anything about the QR code itself, so it's worth testing the exact link on a couple of different phones and, ideally, both iOS and Android, before finalizing anything for print.
File size and format matter more here than they might seem to. A long, uncompressed recording takes noticeably longer to begin playing on a slow mobile connection than a well-compressed MP3 of equal length, and someone standing at a museum wall, a gravesite, or a gallery isn't likely to wait patiently through a slow buffer before giving up and moving on. Compressing audio to a reasonable bitrate - fine for spoken word, since voice recordings tolerate compression far better than music - keeps the experience close to instant even on a mediocre connection.
There's a genuine accessibility angle worth considering too, one that doesn't apply to most other QR types on this site: audio descriptions can serve visitors who have difficulty reading small printed text, whether due to low vision, a reading disability, or simply not being fluent in the language a placard is printed in. A museum or historic site that pairs its written signage with an audio version, reachable by the same code, extends its content to an audience that a purely visual exhibit card would otherwise miss entirely.
Privacy is worth a brief thought if the recording contains a real person's voice, particularly for anything involving a memorial, a personal message, or someone who didn't record the content specifically to be shared publicly - once it's linked from a code anyone can scan, it's genuinely public, in the same way a printed flyer is, regardless of how private the original recording felt when it was made.
It's also worth being honest about when this isn't the right choice. If the content genuinely needs visuals alongside the sound - a demonstration where seeing hands move matters as much as hearing an explanation - a video link will serve the audience better than audio alone, since a listener staring at a static screen while sound plays is a strange, incomplete experience compared with either a proper video or, at the very least, a good written description paired with the recording. And for anything meant to be searched, skimmed, or referenced later rather than listened to start to finish, plain text is usually more useful than making someone sit through an entire recording to find one detail.
A small production choice worth considering: keep the recording itself reasonably short and purposeful. A voice message, a curator's note, or a pronunciation guide works well at under a couple of minutes; a listener who scans expecting a quick clip and gets a rambling twenty-minute recording is likely to close it before finishing, no matter how good the content actually is.