Echo Dot link not fully functional

Hi all,
I spend most of my evenings doing admin on my computer & (with help) have managed to link my external hard drive with my music to my livebox. This enables me to play music via my computer to my Echo Dot 4, which has surprisingly good sound quality for such a small speaker.
But it is in Mono & I would like to play in Stereo.
To this end I purchased another Echo Dot 4 & linked them. Now I get stereo when I ask it to play a radio channel.
My problem is that when I play music from the external hard drive it still only plays through one of the Echo Dots.
Can anyone tell me what I am doing wrong?
I have a pair of cheap Bluetooth speakers from Lidl that have linked & play together but at 5.99€ each the sound quality is not particularly good…

Can the Livebox actually stream stereo over the SMB share ?

I’m no expert on Alexa (nor a fan) but I did notice this reference which in the left pane also mentions this help sheet.
Don’t know if they are significant to your situation though…

Yes. If I ask it to play Radio 2 I get this in stereo but if I select music from the hard drive it only plays on one.

Maybe this thread is of help?

Thanks for this. I am using bluetooth from the laptop to play my own music.
The two Echos play in stereo from the internet OK.

Seems like it is a pretty common problem with Echo Dots. Might be firmware version based.