Resampling signal in MATLAB
To add to Famous Blue Raincoat's comment: the process of upsampling a signal aims to increase its sampling rate without affecting the signal otherwise. In your example, outputting x or resx through your sound card should produce exactly the same sound. Upsampling is useful when you need to match sampling frequencies between different systems without affecting the signals otherwise.

Likewise, downsampling aims to modify the signal as little as possible. All frequencies below the new Nyquist frequency will be left untouched.