I'll use this answer to provide some information that's mostly orthogonal to what Phonon said.
As Phonon pointed out, the speed of sound depends on temperature, not pressure. It's cold on the top of high mountains, so the speed of sound would tend to be lower. Some mechanisms for sound production have a frequency that depends on the speed of sound, and others don't. In the former category, we have wind instruments, which act like resonant air columns, and the human vocal tract, which is complicated but can be understood to zeroth order as a Helmholtz resonator. So one's voice is lower when the air cold, and I think this fact is well known to, e.g., professional singers. In an orchestra, we have some instruments that act like air columns (brass and winds), and others whose pitch is roughly independent of air temperature (strings). As an orchestra plays, the wind instruments warm up and their pitch rises.
Do humans notice these differences in sound? Does the pitch of tones change noticeable?
The highest I've ever been is 5900 m. It was certainly cold, so most likely the pitch of our voices was a little lower than normal, but it wasn't noticeable.
The other effect that one might expect to notice would be that sounds would appear more faint because of the poorer coupling of the radiator to the air and of the air to the receiver (e.g., eardrum). This has nothing to do with temperature or frequency. An easy example to imagine is a speaker. The speaker has a surface (the "speaker cone") made out of something like cardboard or rubber. As the cone vibrates, it excites vibrations in the air. If there's less air, then this excitation is less efficient, because there is less for the speaker to push against. The same thing happens at the eardrum or at the sound-sensitive surface of a microphone. For these reasons, one might expect that human voices would appear softer than normal at high altitude.
The reality is that this has not been a noticeable effect at any altitude I've ever been to. Basically it tends to be windy at high altitude, especially on passes and summits, so if it's windy, it's simply hard to hear for that reason. If the air is calm, then these are very quiet places because they're out in the wilderness, so you can easily hear over long distances.
I'm sure it's true that on the top of a tall mountain the intensity of the sound waves from people's voices is down by several db compared to normal. However, I think there are two facts about the ear-brain system that make this is not noticeable. (1) The physiological sensation of loudness is extraordinarily compressed, in the sense that your ear-brain system takes many orders of magnitude worth of sound intensities and makes them into a perceptual range that doesn't seem so broad subjectively. (2) Psychological sensations are easy to judge when there's an immediate comparison, i.e., in a relative context, but much harder to judge in absolute terms. Think of the eye exam where they flip lenses in and out and ask which is clearer.
One tends to notice these effects very clearly in the classic classroom demonstration where we play a sound through a loudspeaker from inside a bell jar and pump out the air. The effect is very hard to notice until the vacuum inside the bell jar gets quite good (maybe 90-95% vacuum). When I do this for a room full of students, they generally don't notice any difference at all, even at 95% vacuum, until I quickly let the air his back in, and then they can hear the relative loudness more easily because it's a rapid comparison.