Euphonic sound changes
These are changes made to the sound of Japanese words in order to 'sound better'. For example 匹 ひき is a counter used to count animals. However if you take いち (one) and ひき (animal counter) and put them together what you actually get is いっぴき not いちひき.
つ and ち are often transformed into the 促音, 'small つ,' in this way. See how the first mora of ひき is changed by the addition of the little 'o' (半濁音). With 何匹 (なんびき) the first mora is changed by the addition of a " (濁音). Those changes are also very common.
Because these changes are regular and common they are not generally shown in kanji dictionaries. 匹 will only have ヒツ and ひき shown as a reading even though it will often be read as ぴき or びき.
Another important point is how the mora つ and ち are treated when they are changed. When they have " added they become づ and ぢ but those sound like ず and じ. When a word is spelt with づ or ぢ you can generally assume that there has been a euphonic change from つ and ち.
An interesting, if not very nice, example of an exception is the kanji 痔 that has the reading ぢ.
Casual sound changes
There are many casual ways of changing Japanese that involve sound changes.
- では -> じゃ ではまた -> じゃまた
- Vては -> Vちゃ 行ってはダメ -> 行っちゃダメ
- Vでは -> Vじゃ 飲んではダメ -> 飲んじゃダメ
- の -> ん and うち -> ち ぼくのうち -> ぼくんち
These are common in informal speech but should be avoided in formal written Japanese.
[http://www.jekai.org/entries/aa/00/no/aa00no02.htm Jekai on 濁音 and 半濁音)