I'm about ready to return the phone for this same reason. I doubt exchanging it is going to make any difference.
Something is not right with the notification sounds coming out of the bottom speaker on the S9+. They are WAY too soft and muddled. I have my Note 8 sitting next to me and the same notification sounds are loud and crisp at a much lower setting. The S9+ even at max volume is way too soft. However, playing a podcast or youtube video the S9+ sounds great compared to the Note 8. Much Louder and clearer. The difference is notification sounds are only coming out of the bottom speaker while media playback is coming out of the earpiece speaker AND the bottom speaker at the same time. The poor quality sound is when the bottom speaker is outputting by itself.