1 Paris: This is interesting.
1 Alien: Garraken in Tespera.
2 Paris: What we’re hearing is translated. This alien is actually pronouncing completely different sounds.
3 Spanners: So the computer is translating alien syllables into pseudo-English words that make no sense?
3 Paris: Yes.
4 Spanners: Yep, that does sound like typical software.

I believe it is quite likely that intelligent aliens would primarily use sound as a communication medium. Sound is easy to produce, it travels reasonably well within an atmosphere, it's easy to detect, it travels through some objects and around corners so you don't need a direct line of sight to the source, it works in the dark, and it's easy to modulate in ways that can carry relatively dense information.

Some sort of sign language could also be used (and of course is in human society, in cases where sound cannot be used), but this has the disadvantages of requiring illumination and line of sight. You can't use it in the dark, or around corners, so it's less generally useful.

Communicating by flashes of light is possible, and used to some degree by animals such as fireflies. Biologically it seems not too difficult to evolve a system for communication by light, although fireflies are limited to using theirs in darkness. An active light-based communication system capable of working in daylight is another matter entirely, and may be too much to ask from biology. Then there's the question of how densely you can pack information into flashes of light. Computer systems can handle enormous amounts of data sent over optic fibres by nanosecond pulses of light, but biology has limits to how fast you can detect a blinking light signal - think essentially Morse code. Looking up some numbers, sending/receiving about 20 words per minute seems to be accepted as a "skilled" speed for Morse transmission by sound. Visual Morse flashes would be a little different, but I expect somewhere in the same ballpark, as this translates to about 10 dots/dashes per second, which is getting close-ish to human flicker fusion frequencies. I don't think you could go much higher before people would really start to have difficulty. Compared to normal speech at around 120 words per minute (or rapid speech such as sportscasters at around 160 words per minute), a light-based biological communication system is clearly slower for humans, although it's possible it could evolve to a higher-bandwidth state in an alien species. Still, it has a lot of difficulties to overcome.

A scent-based system is possible, and is used by insects that release pheromones. However, such a system relies on air currents to carry your message, and you have no control over which direction it goes - someone a kilometre downwind could get your message while someone standing right next to you on the other side doesn't. And communicating nuance and meaning in a rich way with scent would be very difficult. Imagine producing and then smelling and interpreting smells that communicate a page of text accurately, in the correct order.

And then there's touching and tasting, which again can be used as communication channels but are obviously limited by physical proximity.

Sound is a good medium with a lot of advantages, and many, many animals have evolved to use it to communicate in some form, so an alien biology, on balance, could easily end up doing the same.