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Pronouncing Spanish based on the written word is much simpler than pronouncing English based on written English. Each vowel represents only one sound. With some exceptions (such as w and x), each consonant also represents one sound. Many consonants sound very similar to their English counterparts.
As the table indicates, the pronunciation of some consonants (such as b) does vary with the position of the consonant in the word, whether it is between vowels or not, etc. This is entirely predictable, so it doesn't really represent a breaking of the "one letter, one sound" rule.
The University of Iowa has a very visual and detailed explanation of the Spanish pronunciation.
Here is another pages with links to the audio files of the letters.