Spirits are all around us, and can only be seen when one looks at the auras about them, or Astrally Projects outside of his/her own body. Our Astral Bodies' eyes can see them, just not our Physical eyes.
Spirits can communicate with us, but cannot appear very well if at all. Most people experience them everyday without knowing it. Have you ever seen a white, black, purple, or cyan-ish shadow whiz by out of the corner of your eye when there is nothing that could have otherwise caused that? It may very likely have been a Spirit.
How Spirits Communicate
The way that Spirits communicate with us is through vibration. I know that may sound strange, but it's actually not. When you are hearing your spouse, co-worker, or teacher speak they are moving their vocal chords. This creates vibrations of various pitches and wavelengths in the air around them. This vibration is then received by your ears which translate this vibration into electrical signals which our brains interpret as the intelligible, spoken word that we, as physical entities, are all familiar with.
With dogs the sound works the same way, but they are speaking an entirely different "language" (range of frequencies) so-to-speak.
Spirits are like dogs. Have you ever watched two dogs look at each other, give no indication with body language nor make a sound, and then run off together? This is because they speak in frequencies that the human ear cannot perceive. This is similar to how Spirits speak. Everybody is capable of understanding this language, it just takes practice to develop one's ears to accept such frequencies (in most Spirits' cases low frequencies).
This is why you may encounter "strange, low, unintelligible voices/moans/vibrations" when a Spirit is around. Once you train your spiritual body to accept and interpret these vibrations, you can actually understand what they are telling you plus or minus a few words. The strange thing is, you won't actually hear any noises.
This is because our ear drums do not pick up frequencies as low as these, so they pass straight through to the brain for interpretation. (This is one theory on how telepathy works.) If you ask the Spirit to speak up, or speak more clearly, oftentimes you will hear loud-ish, low vibrations shortly after if the Spirit feels like.
To speak to Spirits, you must do so Astrally, as well as to hear them competently. This is one theory on how dogs speak to each other Astrally/telepathically, but if you ask me, however, the verdict's still out on that one.