My company just got me a new Samsung Galaxy S5.  I was using an iPhone 4s. 
I have read some reviews of the S5 that say the audio is OK but not great.  
To my ears, it sounds great.  It will not get quite as loud in volume using earphones as my 4s but it is still plenty loud to me.
I have reviewed the HTC 1 M8 and have seen good feedback on its more powerful audio amp and stereo speakers.  
 
What do you think of the S5's sound quality?
 
Thanks.
 
Rimshot
 

I had the Galaxy S2, but now have the iPhone 5s (funny how it works that way). Anyway, I had no issues with sound on the S2, I generally used the Plantronics Backbeat 903 Bluetooth headset with the S2 and sound was quite good. I assume they did not butcher the audio in subsequent models. I never hooked it up to the stereo though, but can't see why there would be issues, other than being MP3 source.

One thing I did have an issue with the S2, and hope was fixed by the S5, was recording video of a live band that had some volume. The audio would be completely distorted and unlistenable. So if that is something you might ever want to do, I would test that feature before assuming it will be acceptable.

Thanks for your feedback Ken.