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.
Thank you for the answer. Although, I have another doubt: If it is subjective, then why do you say it is almost vowels (in the spanish languague)? Are you saying that, basing yoursef on the mayority of points of view on the subject, which will tell you that vowels or almost any vowel is louder? (sorry if I sound confusing, I'm new at this). Mar 3, 2021 at 16:07
2"Subjective" means "that's how it seems to me" (the experimental subject). It does not mean arbitrary or uncaused. One of the goals of psychoacoustics is to develop a theory of what objective (usually acoustic) measures best correlate with "loudness". It comes down to saying "Here are two sounds. Which is louder". I conjecture what the results of such a study would be, but I have not conducted the study and I don't know of any study which tries to. My conjecture is based on a tiny bit of knowledge of Spanish, and more knowledge of acoustic properties.– user6726Mar 3, 2021 at 16:14