I have been to weddings where, during the recitation of the seventh bracha under the chuppah, the congregation sings "asher bara...". Sometimes it is along with the one reciting the bracha and other times he waits for them to sing it and then repeats it himself. Is this a halachic problem either because the chassan and kallah have not heard the entire bracha from the one saying the bracha or maybe because the pause in between is a hefsek?
There is no question that the person saying the beracho should wait. The Chosson and Kallah need to hear every word of the beracho since it is being made on the wine for them. It is better that a minyan hears every word too. Thus, the standard practice among gedolim who say sheva Berachos is to wait until the singing stops and then continue. (Source: Numerous weddings I've been to, where the Rav saying the sheva Berachos is an expert in hilchos Tefilla and berachos and is a Baal Tefilla for Rosh Hashana/Yom Kippur). It is implied from the חשוקי חמד ברכות דף מ עמוד א that even if the chosson makes an interuption one can be lenient, and thus the pausing due to the person waiting for the singing to stop would not be considered a hefsek.
There are 3 main problems with the person singing along:
- It is a hafsaka. This is much worse than waiting and not saying anything.
- The tune tends to repeat the words a few times over. This definitely invalidates the bracha.
- Sheva Brachot require a miniyan to hear and answer amen. That is not possible when 200+ people are singing.
A VERY late edit: At my wedding I was told not to sing along because the chattan also has to hear the whole beracha. The "mesader kedushim" was Rav Goldvicht zt"l so I suppose he counts as a good source.