Bluetooth earphones suddenly extremely loud and distorted on MacBook Air M4 after macOS 27 update

The troubleshooting already performed is useful and reasonably thorough. In particular, removing and re-pairing the earphones, resetting the earphones, testing multiple applications and audio sources, disabling the Music EQ, and comparing the behaviour with the same earphones on another Mac are all worthwhile diagnostic steps.

The fact that the earphones behave normally on another Mac also makes an obvious hardware failure of the earphones less likely.

The main limitation is the comparison itself. 

The test compares macOS 27 on one MacBook Air M4 with macOS 26 on a different Mac. Because both the computer and the operating-system version changed, the test cannot determine whether the difference is caused by macOS 27, the particular M4 Mac, its Bluetooth hardware or firmware, a device-specific configuration, or an interaction between that Mac and the earphones. In other words, the comparison establishes that the problem follows the particular Mac/macOS 27 combination, but it does not isolate macOS 27 as the cause.

That distinction is important because Bluetooth audio behaviour depends on the interaction between the Mac and the Bluetooth device. Apple, for example, documents that Bluetooth headphones can switch between different operating modes depending on whether their microphone is being used, with corresponding changes in audio quality and volume. (Apple Support) Apple also notes that available audio options can vary depending on the Mac and the connected device. (Apple Support)

The observations therefore support saying that something about the macOS 27/MacBook Air M4/earphone combination changed, but they do not yet establish that macOS 27 introduced a general Bluetooth-audio defect. Likewise, suggestions involving codecs, Bluetooth profiles, Audio MIDI Setup, or audio processing are reasonable areas to investigate, but they should be presented as possibilities rather than identified causes unless testing demonstrates a specific change.

A stronger comparison would involve controlling more of the variables. For example, testing the same earphones on another MacBook Air M4 with macOS 27, or testing the affected M4 Mac with another pair of Bluetooth earphones, would help distinguish an operating-system-wide problem from a Mac-specific or earphone-specific compatibility problem. Testing the affected Mac with the same earphones under another user account or after confirming the relevant audio-input/output configuration could provide additional evidence as well.

So the work already done should not be dismissed.

It has established a meaningful and reproducible difference and has narrowed the investigation.

The point that needs to be softened is the leap from “the earphones work on macOS 26 but not on this Mac running macOS 27” to “macOS 27 is causing a Bluetooth audio-processing, codec, profile, or volume-management bug.” The former is supported by the testing described; the latter still requires further isolation.

One additional correction to the original reasoning: I would not characterise Audio MIDI Setup as an established explanation for excessive Bluetooth volume. Apple says it can provide additional volume options depending on the Mac and device, but that is different from documenting it as the cause of this particular behaviour. (Apple Support)

The most scientifically useful next test would therefore be cross-testing the variables rather than changing more settings: affected M4 + different Bluetooth headphones and another M4 + the same earphones. 

Those two tests would tell you considerably more about whether the problem is associated with the Mac, macOS 27, or the specific earphone/Mac combination.

Now working again.

I performed a reboot and used Mole from GitHub to clear out all caches etc.

I experienced the similar issue with my Macbook Pro 16 M5 Pro running 27.0.1 yesterday, I was using my Sony ULTwear headphones connected via Bluetooth and trying to listen to YouTube Music. The sound was quieter and awful - no bass etc. I tested with Safari and Firefox, same result. My speakers plugged into headphone jack are working correctly. Headphones were working perfectly on Tahoe and are working on my iPhone running iOS27, so I suspect a macOS bug :(

This is happening to me, too.

MP3s played from my library in Apple Music on my laptop (Apple M4, Golden Gate) are crackling, 'sound too loud', and are distorted even if turned down on my JBL Bluetooth speaker. This problem just started (mid-September 2026).

The same songs (MP3 files) played via the same Bluetooth speaker aren't distorted from my iPhone.

This is only happening on the laptop --> Bluetooth speaker.

The distortion is not occurring with other audio on the laptop (streaming, etc) as far as I know or have noticed.

I have verified the firmware on the JBL is up to date.

This is almost certainly related to the update to MacOS 27.

Help?

Thank you.