Music Quality on Fenix 6X

I use my own mp3s podcasts and audio books using the watches unbuilt system via express....no sync service needed... I don't have any issues with dropouts with my Bluetooth speakers or my Samsung galaxy earpods....

I also upload these mp3s in 320kbps for better quality....

not sure of the bitrate of my music on my garmin 6s saphire as I sync a dedicated playlist from spotify but as far as I can experience, sound is rather good on my sony mdr10bt, my jbl. yes it's certainly not aptx/ldac but it's consistent and not cutting. I tried a few times during a gps activity and did not have issue either. now most of the time a use is having issues with a bt headset, it's with samsung galaxies.

I have the 6 sapphire and I also notice worse audio quality compared to my iPhone, especially in the treble. I have uploaded mp3 and aac files in maximum quality, also from Spotify and the result is the same. I have opted for resignation. Perhaps Fenix 7 improves this aspect... fingers crossed.

i have a few mp3 stored on my fenix 6 and quality is the same as the iphone using over the head headphones.

Then you got the wrong headphones for your phone if they don't support Apples high quality bluetooth codecs. 

I believe the Fenix 6 only supports the standard SBC codec, not higher quality codecs such as APTX

In other words, you can't 'fix' this.

I use Amazon on mine, and it sounds fine. It's not Ultra HD or ATMOS, but it sounds fine. I use Bose Open Ear headset. Do you use Amazon's Music that comes with Prime, or have you upgraded to Unlimited? Both can be used, but maybe the download resolution is limited on one vs another.

It looks like the error is due to the quality of your music files on the watch. Transferring music directly from streaming services to external devices is limited to a certain quality level depending on the settings. To save time and capacity of the transfer via Wifi or Bluetooth, a setting with lower quality will surely be the reason.
Just compare two identical mp3s with high quality (320 Kbs) with each other by loading them directly via the explorer on the watch.
By the way, I can't find any major flaws with my Fenix and the music quality. I also do not use streaming services like Amazon or Spotify.

I have a decent amount of music on my device and I cannot tell the difference using the same bluetooth headphones between it and my phone, on which the same file resides.  MP3s are inferior to FLAC, but bluetooth is inferior to wired by quite a bit in terms of noise floor and such, so I doubt it matters with wireless headphones.  It sure does if you plug in to your phone and use a good pair of studio cans, or even better, a quality DAC off the USB port.

I suspect your transfer from the streaming service is degraded and the watch is not responsible.

I have tried several headphones with my Fenix 6S Pro and all sound terrible (both with local files and Spotify synced songs). I think it's due to Garmin only supporting SBC, and a really low bitrate too. Probably a limitation of the hardware of the watch, a higher quality encoder would use even more battery power. Thus I never use the build in Music feature, just the phone controls. Bought it for the mapping features anyway :D

Switching to Mono instead of Stereo helps to reduce perceived quality loss, I hope it is using the available bitrate better for Mono atleast.

Here is a test I made where you can listen and compare for yourself: https://djazz.se/nas/audio/garmin-audio-quality-test.zip (28 MB)