There are 21 initials and 33 finals in Mandarin romanization (there are five more finals, but they share the same symbols as other finals). These combine to form about 420 different sounds. A word consists of (initial) + final + tone. In some cases, the initial can be omitted.
To hear a particular initial or final, you can use the Mandarin Sound Chart.
ㄅ ㄆ ㄇ ㄈ ㄉ ㄊ ㄋ ㄌ ㄍ ㄎ ㄏ ㄐ ㄑ ㄒ ㄓ ㄔ ㄕ ㄖ ㄗ ㄘ ㄙ
Below is a list of the bopomofo finals. Notes on more difficult sounds follow below.
ㄚ ㄛ ㄜ ㄝ ㄞ ㄟ ㄠ ㄡ ㄢ ㄣ ㄤ ㄥ ㄦ 一 ㄨ ㄩ