2 letters, 1 sound: Digraphs

Two letters (in written form) can represent a single sound. These are known as digraphs. You probably know them as ch, sh, th, and the like.

English doesn't have a single letter to represent the initial sound ("ch") of the word child, so it must use two letters. Recognizing a digraph is one of the many difficulties in learning a language. For example, take the "ch" sound as mentioned above. One CANNOT simply read it as "a 'c' sound followed by an 'h' sound" - it is a completely different sound altogether.

Here's how the some of the digraphs are represented in phonetic transcription (e.g. using IPA):
Most of the time, only one symbol is used because it represents just a single sound, even though it takes two letters in the written form.

The "sh" sound (as in shell) is represented by [ʃ].
The "ch" sound (as in church) is represented by [tʃ]. Two symbols are used but it is still a single sound. It is actually a combination of the "t" and "sh" sounds. Try making a "t" sound and a "sh" sound simultaneously and the result will be a "ch" sound.

In some dictionaries and texts, [š] and [č], are used to denote the "sh" and "ch" sounds, respectively.

There are many more digraphs, and they also exist in languages other than English.

Vowels and musical languages

Even though we only write 5 vowels, English has as many as 15 vowel sounds! (this can vary depending on dialect)

What's useful about the consonants and vowels of a language?

Although French has nasalized vowels, making them difficult to pronounce for many English speakers, French is a very musical language. This is because words tend to flow from one word to the next with no pause in between them. In situations where there would be a pause, French requires that sounds be added or words changed. This is known as liaison.

Spanish has a lot of vowels and could be considered a musical language.

Even more so, Italian is considered the traditional musical language. This is due to the fact that most words end in a vowel. Not only does this make it a very suitable language for opera, it also means that once you are familiar with its rhythms, it is a comparatively easy language to pronounce. Unlike English, Italian has predictable pronunciation. Every letter has a specific sound, and there is not much difference between Italian spelling and pronunciation.

On the other hand, German does not really sound musical. Long word lengths, hard sounds for the consonants, plus many consonant clusters make the language difficult.