hmm, I don't know about the original's sound quality, though I heard about how the international version has very compressed sound due to some hardware problem (the way the soundcard is installed), GSMArena's audio test also found high distortions. They said the One X+ has much less distortion and better overall sound quality but volume is not as high. These are without Beats Audio.
I can say the sound is very good on the One X+, but not as good as my iPad 2. Soundstage not as good, but still very good; the bass is very tight without Beats and the treble is clear. Some people on other forums are saying the sound is good or bad already but it's all subjective.
I don't think there's an amplifier for the headphone jack still. That's only the 8X and the Droid DNA. Both of those phones sound good according to GSMArena, but they have a curved frequency response not flat like the One X+. Some people are saying they increased the voltage of the headphone jack though I cannot find any specs on that. I'm still trying to find a way to boost up the volume without Beats (equalizers don't seem to work at boosting volume as the phone seems to automatically lower the volume first when you apply an equalizer). At max volume with my Panasonic HTX7 it's still very quiet so I use Beats Audio for that (though it seems a bit imbalanced at times, not bad actually). I turn off Beats when I use my in-ear headphones (Klipsch S4) because the volume is just fine and the sound is very good with these headphones.
I'm not sure about the sound quality in comparison to the One X Snapdragon S4 version, because originally that one had a lot better sound quality than the Tegra 3.
The external speaker is pretty good though it does have this "compressed" feel to it (I don't mean actual compression but as if the sound has been equalized, which isn't that bad).
Facebook
Twitter
Instagram