XDA Senior Member otonieru has come up with a modification which makes it possible to enable stereo sound output on any Galaxy S8 or S8+ smartphone. The mod enables the phone’s earpiece as a secondary speaker to achieve stereo output. Of course, nothing compares to having two equally sized stereo speakers, but it’s still better than having just one speaker. Check out the linked thread for more details.
Stereo Speaker Mod for the Galaxy S8 and Galaxy S8+