Learn Italian: La pronuncia ha molti più suoni di quello che credevo. - The pronunciation has many more sounds than I believed.

We use di quello che (or di quanto) because we are comparing a noun (suoni) to a whole clause with a conjugated verb (credevo). If you were just comparing two simple nouns (like "more sounds than words"), you would use che or di. But to say "than I believed", you need this special clause-level structure.

Could I use di quanto instead of di quello che in this sentence?

Yes, they mean the same thing and are used for the exact same purpose. di quello che is a bit more informal and very common in everyday speech, while di quanto sounds slightly more formal or elegant.

I thought verbs like "credere" required the subjunctive. Why is it credevo (indicative) here?

This is a great catch! While di quanto almost always requires the subjunctive (di quanto credessi), di quello che is more flexible. In everyday spoken Italian, it is very common to follow di quello che with the normal indicative tense (credevo), as seen in this sentence.

Why is it molti più suoni and not molto più suoni?