I cannot. There are no Apple Stores in my country. Authorised resellers here dont have any return policy.
But didn't you mention that you traded in your s10+ to get the 11 pro? I'm sure you can do it the other way around, too.
If the 11 pro's speakerphone sound quality is not up to your standards as you noted, then I don't see why you would hold on to it. If you suspect defective hardware, take it back for repairs or replacement. If you believe it's just poor quality and it bothers you, trade it in.
If this is, in fact, not a big problem for your every day use and you find the 11 pro otherwise superior to the s10+ for your needs then I'm positive the regret you're feeling will pass and you'll be happy with the iPhone.