I've brought this up in the past and Samsung faithful just downplayed it and stated majority use headphones/earbuds anyway when listening to music so it's not that big of a concern. I never understood why manufacturers placed speakers on the back of the device. These are smartphones where people look at the screen while using it. Speakers should be on the front.
It can be done successfully. The Lumia 1520 for example has it's speaker on the back, and contrary to the assumption this would muffle or restrict the sound output, the actual positioning / angle and air space around the Lumia 1520's rear speaker actually enables the device to turn any surface its on (a table or pressed against a wall) to act like a woofer and amplify the the sound resulting in louder volume when on a flat surface than when speaker is facing upwards.
So there are examples of exceptions to the the rule, and it's down to manufacturer.