My only problem with EVP is that the mind can make sounds from noise, ESPECIALLY if it's something you want to hear. My opinion, and not from a religious point or anything, perhaps you should look for other signs she's there. In a dream, thoughts, or every day life, you may notice something unusual or something that reminds you of her, to me, I believe this is the way they can let you know they are OK but that's just my opinion of course.
A quick example, I have an epson printer that when printing, makes the exact noise to sound like, "HELP ME". It's plain as day, not an entity but still funny so when it goes on the fritz, we understand.
I do believe we can hear sounds from years back or whenever but not from spirits. Much like they can, by tracing light waves, see in the past, the same can be done with sound. Not saying it's not possible but if there was actual proven clear communication through EVP, then that would be different. I do keep an open mind though.
I am still very sorry to hear what happened and I have lost many in my life as well and it seems things will happen every so often that are too coincidental to let me know they are still with me to some degree.
Paul