Lightly water damaged iPhone SE - issues with corrosion and speaker

I recently bought an iPhone SE from eBay in a lot of devices. The iPhone SE works, except there is corrosion near the battery connector, and the loudspeaker does not work.

The corrosion is in a small area to the right of the battery connector, it doesn't seem to cover any important components.

I was worried the audio IC was busted, but the earpiece and headphones work, so it seems to be a problem with the speaker itself.

I intend on saving the phone, because the water seemed only to affect the lower third of the phone. I replaced the loudspeaker with one from a working iPhone 5S, but the issue still continues.

In short, how do I tackle these problems and prevent the phone from further damage?