As the Steipler Gaon points out (Kehilot Ya’akov, Pesachim 7, and Megilah, 4), in acknowledging blowing the shofar or hearing the megilah as legitimate, if theoretical, possibilities for a berakhah, the Ran is touching upon two disputed issues:
a) It is questionable whether the blowing of the shofar is a mitzvah. The Rosh (Rosh HaShanah IV, 10) quotes a dispute between Rabbeinu Tam and the Behag as to what action should be named in the berakhah over the shofar: hearing or blowing. According to the Rosh, their argument centers upon the question of what is the actual mitzvah of shofar, hearing or blowing. It is unlikely that the Ran dismisses hearing as being part of the mitzvah. If, however, he believes it theoretically possible to recite a berakhah upon blowing, it seems he believes that both hearing and blowing are parts of the mitzvah together. (That is also the view of the Kapot T’marim, Rosh HaShanah 29b; see also Resp. K’tav Sofer, O.C. 104; Emek Berakhah, p. 79; Resp. Binyan Av, I, 24)
b) The possibility of reciting a berakhah on hearing the megilah implies that the act of hearing is significant in and of itself. This, however, may depend on how one understands the concept of shomea k’oneh (hearing is as if reciting) the subject of much analysis. If the notion is understood to mean that hearing a verbal mitzvah an equally valid alternative to reciting it, then indeed the act of hearing can be termed a mitzvah (although not the primary mitzvah act, and thus not the ideal berakhah option). However, if shomea k’oneh means that the listener fulfills his obligation through the recitation of the speaker, with whom he is partnered, then both parties are actually fulfilling the mitzvah through the act of the single recitation; the hearing itself would not be termed a mitzvah.