The simple reading of the gemara is that Rabba’s peshat is accepted and in explaining that the beraysa that says it is assur to to put wheat in a Rechayim is according to Beis Hillel. This then means that the issur is because of hashmaas kol and such is the pesak of Rabbeinu Chananel. However, Rabbeinu Tam has a different reading of the gemara. He understands that Rav Oshaya accepted the reading of Rav Yosef in the beraysa, i.e. the words מאן תנא שביתת כלים דאוריתא are not discussing the opinion in general, but the beraysa we had just discussed. Thus Rav Oshaya agrees with the peshat of Rav Yosef (and he is the decider of pesak!), in which case the issur of hashmaas kol is not mentioned there, rather the issur by rechayim is due to שביתת כלים. This would mean that we do not have such an issur of השמעת קול learned out from the beraysa and perhaps no such issur exists.
As it turns out, the issue of what the pesak is in our sugya is a major machlokes rishonim, with the Baal Hamaor agreeing with Rabbeinu Tam and Ramban disagreeing. The Rambam omits this din and the Rif seems to be meikil about hashmaas kol as well, though he quotes the opinoin to be machmir due to hashmaas kol. For this reason, the Beis Yosef writes that we can be meikil in hashmaas kol and codifies this in the Shulchan Aruch as well (רנב:ה). The Rema starts by agreeing with this and then writes ויש אוסרים.
For this reason under various circumstances heterim are given even for things that make noise, especially in cases of financial loss. One significant one (and important if you want to do chazara early shabbas morning) is for an alarm clock, as the Rema writes “אע”פ שמשמיע קול להודיע השעות בשבת כי הכל יודעים שדרכו להעמידן מאתמול ”
Rav Meir explained in shiur that the issue of hashmaas kol is based on אוושא מילתא, which means a zilzul in shabbas. What creates the zilzul – if it is the noise itself, then why should the clock be allowed just because people know you set it up before? I think the tzitz eliezer in chelek 4 siman 31 (http://hebrewbooks.org/pdfpager.aspx?req=14503&pgnum=169) explains this idea. He says that the אוושא מילתא is still connected to whether people will think this is something you may have decided to do on shabbas in violation or obviously did beforehand. In his case, (he is dealing with watering seeds on shabbas where the sprinkler was turned on before shabbas), he feels that that people will realize that you needed to set it up beforehand (due to the urgency of that case), so maybe the idea is the same with the alarm clock – that it is obvious to people that you set up the alarm clock before and therefore the noise itself is less of a zilzul.