>>221
What you're referring to involves several different phenomenons, but, in the case of your example, it's par of 連濁 (rendaku), which refers to a change of the first sound to 濁音 (dakuon) in the second component of a compound word. For example, "bi" in your example is the dakuon which was originally "hi."
This can also occur in a compound of two different words such as 株式会社 (kabushiki-Gaisya), which is 株式 (kabusiki, join-stock)＋会社 (Kaisya, company). It's too difficult to explain rendaku in detail, but in the case of compounds of the same kanji as in 人々, it's simple:
If the first sound of the kanji is k, s, t or h, then the corresponding sound of the second component changes to g, z(j), d or b, respectively.
For instance, 細々 (hoso+hoso) is pronounced hosoboso while 易々 (yasu+yasu) is yasuyasu.
As I said, there are many other things if you're talking about the sound changes of the second kanji in a word in general. For example, a bit complicated rule about rendaku says 山川 is pronounced yamakawa if it means "mountains and rivers," and yamagawa if you mean "a river running in a mountain." 云々 (un-nun) is an example that doesn't fall into rendaku. It may look similar to 人々 and such but actually follows another rule (and the pronunciation of the second kanji is altered).
That said, every rule has exceptions and you can get by with the simple redaku rule. In fact, no one cares if 山川 is mispronounced, and others like 云々 are rare so it's easier to remember each word than master the rules.