  Sound attenuation and internal friction coefficients are calculated for a
realistic model of amorphous silicon. It is found that, contrary to previous
views, thermal vibrations can induce sound attenuation at ultrasonic and
hypersonic frequencies that is of the same order or even larger than in
crystals. The reason is the internal-strain induced anomalously large
Gr\"uneisen parameters of the low-frequency resonant modes.

Condensed Matter