How do Chinese speakers pronounce unknown characters? I'm wondering what do Chinese speakers do when they are reading aloud and encounter a character they don't know (sometimes it happens, isn't it?).

In phonetic languages, there is a possibility to read an unknown word correctly, but in the ideographic ones you have no idea on the pronunciation. Meaning is also hard to guess, because that unknown word is likely to be a very narrow term. E.g. it's difficult to guess what the bleak or anticlinorium are, even having the context.

So what is the most common way for native Chinese speakers to read aloud unknown words?
When encountering unknown characters, native speakers often refer to dictionaries, which have pinyin of the unknown character.

While it is hard to guess the exact pronunciation of unknown characters, there are a subset of Chinese characters, called phonogram(形声字), whose pronunciation is related to part of the character. For example, 蛛(spider) has the same pronunciation with 朱.