。
It's easy to understand the way she speaks in a clear tone.
Literal
She [topic-は] clear [adjectival-とした] tone [with-で] speaks [nominalizer-の] [subject-が] easy-to-understand.
のが nominalizes the speaking-clause and makes it the subject of わかりやすい — 'the way she speaks (in a clear tone) is easy to understand.' The same content can also be cast with ので ('because') connecting two clauses ('she speaks clearly, so she's easy to understand'); the choice shifts the focus from her manner of speaking to her as the easy-to-understand entity, and both phrasings are fluent Japanese. はっきりとした口調 ('a clear tone of voice') uses the productive mimetic-derived adjectival pattern: a mimetic + と + した stamps the modified noun as having that quality.