Isn’t the iPhone 8+ supposed to be waterproof?

iPhones have a degree of water resistance, but are not entirely 100% waterproof.

About splash, water, and dust resistance of iPhone 7 and later - Apple Support

There is no speaker on the left side of the bottom edge only a microphone.

It may sound like it since the right speaker is very close to it, but no sound is supposed to come out of there as its just a microphone.

If you read Apple's description of "IP67" they claim that it's water/dust resistant to a certain standard, but that they are "not permanent conditions". They indicate that iPhones are subject to normal wear that may decrease the water resistance over time.

No iPhone is waterproof.

On iPhone 7, 7 Plus, 8, 8 Plus there is only one speaker at the bottom, the other opening is a mic.

The second speaker is at the top close to the front camera.