The volume of the ambient noise seems to be higher than speech. I'm having problems making out the characters' voices. Is there a way to change the volume or another solution?

Where in game are you experiencing this ?
If you mean the characters in the blue/red books in the library, the 'static' that keeps you from understanding what they're saying will 'dissapear' as you progress through the game.

Right now, the red and blue books in Myst I can't make out any words. I know most words are supposed to be garbled by static, though. In ChannelWood I can only understand about 50% of what the left recording says. The birds, wind, and wood creaking drowns out the voice.

Those speeches from the books will clear up as you go, don't worry.
Ah yes, Channelwood is a bit of a noisy Age ;)
The messages in Channelwood: only one of them is in english, never mind trying to understand the others - they're gibberish, you'll understand their purpose later on, if you haven't already.
I don't know of a way to raise just the speech level, but i can tell you the content of the message in the imager (the one in english). This is probably the only speech in the game that is a bit hard to make out, all the others are easy to understand.
***SPOILERS***
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.
.
Message in the Imager in Channelwood:
"I hope i pushed the right button, my dear brother. What a very interesting device you have here. I'm not erasing anything important, am i ? Remember, he is preparing. Take only one page, my dear brother."

I had the same issue with Real Myst. I found that I had to increase my volume about two - or even three- fold in order to hear the dialog. Hard to manage when you're playing at night with 3 roommates :(
I never found a solution other than popping in head phones so the sudden gush of ambient noise didn't aggravate anyone. Volume up during dialog, then volume WAY down.