Since English is (largely) not tonal, you're not used to the rapid pitch changes required by tonal languages, hence the "alien" feel. For what it's worth, I find Thai quite euphoneous compared to Vietnamese or Cantonese, both of which have even more tones.

Question for HN: if your first language is tonal, what do non-tonal languages sound like to you? I've frequently heard Finnish described as boring/mumbling, because it's both non-tonal and has a very predictable stress pattern (emphasis always on the first syllable).