Tefillat HaDerech is an obligatory prayer asking for protection from the potential dangers of traveling.[1] Since it was instituted in beracha form, we assume that it follows the standard rules for fulfilling the obligations of berachot and prayers.
One of the most basic rules of being yotzei (fulfilling) one’s obligation through listening to another person’s recitation of a beracha, a required text, or shofar blowing is that the other person must be obligated as well.[2] Even the permissibility of answering amen to another’s beracha requires that the beracha be a meaningful one.[3] Therefore, all agree that one is not yotzei and should not answer amen to a beracha that he hears from a recording, as that which he hears from a machine is not significant in this regard.
Does hearing another person’s recitation via microphone have more halachic significance than hearing it from a recording? Almost all poskim agree that one cannot fulfill the mitzva of hearing shofar via microphone, because one must hear the direct sound of a person who is obligated in the mitzva blowing the shofar.[4] However, the ruling regarding hearing the Megillat Esther reading via microphone is less clear. Although one does not hear the actual voice of a valid ba’al korei when his voice is projected over a microphone, hearing the reading via microphone is nevertheless certainly better than hearing it via a recording. This is for two reasons: First, the sound heard from a microphone is a direct transformation of the sound waves produced by the ba’al korei. In contrast, the sound produced by a recording is caused simply by someone pressing a button, thereby causing a machine to read a stored coding. Second, when using a microphone, the sound is heard essentially at the same time as the ba’al korei’s reading, unlike the replaying of a recording. Due to these factors, a lenient position regarding fulfilling the mitzva via microphone is somewhat tenable (although most poskim nevertheless still maintain that one cannot do so).[5]
A gemara[6] indicates that it is not always necessary to actually hear the voice of the person’s recitation, as long as the listener knows what is being said. The gemara discusses a huge synagogue in Alexandria, in which many of the congregants could not hear the sheliach tzibbur; flags were therefore waved to inform those present when to answer amen. Thus, it might seem that one can fulfill his obligation even if he does not hear the actual words being recited. However, Tosafot[7] limits this precedent to cases in which the participants are not attempting to fulfill any mitzva at the time; the gemara’s discussion relates only to their answering amen. In any event, this source does indicate that at least in regard to answering amen, it is sufficient for one to know what is being recited, even without hearing the actual recitation.[8]
Of the different views, the much stronger position is that one cannot be motzi others in mitzvot, including the recitation of Tefillat HaDerech, via microphone. Nevertheless, while few Orthodox shuls use a microphone for reading Megillat Esther, it is more commonplace for people to say Tefillat HaDerech, for others as well, over a microphone. Can the two practices be reconciled?
The answer is that they arguably can be. First, it is generally assumed that the level of obligation in Tefillat HaDerech is of a lower level than that of set tefillot, thus making leniency in a case of machloket easier. Furthermore, when the beracha is recited on a bus, there are often additional grounds for leniency. Rav Ovadia Yosef assumes[9] that whoever could possibly hear the recitation without the microphone can be yotzei even if in practice he hears the recitation primarily from the microphone. While one can take issue with this assumption, it is a reasonable one from an important posek.[10] Applying this to your case of people on a bus, it is likely that most could hear the words of the Tefillat HaDerech without a microphone; they just might not hear them as well. Even if they could hear only most, but not all, of the words, that would often suffice for Tefillat HaDerech (unlike the case of Megillat Esther, for which one needs to hear (or read) every word).[11]
It appears halachically preferable for people to recite their own Tefillat HaDerech along with the leader, especially those who could not have heard the words without the microphone. (One who does so noticeably, when others do not, might run into problems of yohara.[12])
We note that in the case of many driving trips, there is actually a machloket whether Tefillat HaDerech should or should not be said, at least regarding its beracha ending. This means that in some cases, merely answering amen would actually be preferable to saying Tefillat HaDerech oneself, as a possibly unjustified amen is less halachically problematic than a questionably unjustified beracha.[13] Therefore, the common practice (which we prefer justifying when possible) of one being motzi others in Tefillat HaDerech via microphone sometimes has a slight element of advantage.