for IEM's, not all IEM's completely block out sound. The V-moda vibes are an example. Maybe a better wording would be changing the phrase
"They are inserted directly into the ear canal, blocking out external sounds"
to
"They are inserted directly into the ear canal, possibly blocking out external sounds depending on design."
Other words you may want to include: extension (bass, highs), ear bud, sound stage, single ended, balanced output, line out, decibel, attenuation, microphonics, sampling rate, jitter, veil

,