Page "Shofar" Paragraph 17

The Sages indicated that the mitzvah was to hear the sounds of the shofar.

They went so far as to consider a shofar blown into a pit or cave and to decide whether a person who hears the original sound or the echo has fulfilled the mitzvah.

Mishnah ( Rosh Hashanah 27b ).

The Shulchan Aruch sums up that if the hearer hears the reverberation, the mitzvah is not valid.

However, if the listener perceives the direct sounds, he fulfils the mitzvah.

Thus, most modern halakhic authorities hold that hearing a shofar on the radio or the Internet would not be valid to satisfy the mitzvah because " electronically reproduced sounds do not suffice for mitzvot that require hearing a specific natural sound.

However, one should consult a competent rabbi if an unusually pressing situation arises, as some authorities believe that performing mitzvot through electronically reproduced sound is preferable to not performing them at all.