AKAPella is an a cappella group that performs parodies on other artists' songs, often non-Jewish ones as of late, in a cappella for people to enjoy during sefira and the three weeks.
In their albums, they use what I assume to be computer filters to make their voices sound like real instruments. Although if one listen closely it's obvious that it's not the actual instrument, they do such a good job mimicking it at a cursory listening that they have actually printed on some of their CD's "So good it should be assur."
But why isn't it forbidden? Firstly, are computer synths not considered to be instruments? And secondly, even if it's technically permitted, how does it uphold the spirit of the time, in which we are supposed to mourn? We stay away from music as a sign of mourning, and they go ahead and put out a cappella that might as well be real music?