I think vowels but I'm not so sure. I'm neither sure if vocal cords are what makes a phone be more audible and so what makes vowels be more audible than consonants or some of them.
It is almost vowels, but to verify this you would have to conduct a perceptual experiment, since loudness is a subjective ranking of sounds. Things that affect loudness judgments are duration (really short sounds aren't as "loud"), frequency (really low-pitched or high-pitched sounds aren't as "loud") and sound pressure level (the thing that is usually measured). Consonants can have some higher SPL components, the release bursts, but they tend to be very short and high pitched. Click consonants, lacking in Spanish, and particularly loud. Otherwise, vowels score high on the properties contributing to the feeling of loudness.