See the comments on "Aya" for more possibilities for meanings. As for the "ka" part, it could be "beautiful" or "song" or "fragrance".
If it was pronounced Ayaku, it would be spelled that way. Japanese is very phonetic and it's said the way it's spelled, consistently (like Spanish).
Also, the Japanese language doesn't have the syllable "ye", so they'd have to do some fancy work with the characters/syllabary and it would be spelled differently (Aieka) because the "ye" sound when they're pulling it from other langauges (since it doesn't normally come up in Japanese words) is written "ie".
Ayaka is a very popular name these days. It can be divided into two elements, 'Aya' (colourful) and 'Ka' (fregrance), and 'Ka' can be replaced by other naming elements such as 'Mi'(beauty), 'Ko'(child), and so on to make other names 'Ayami', 'Ayako'. I personally love that names.