I have a QSI Quantum Programmer. If you are strictly using QSI sound decoders, whether originally installed (like Atlas locos) or Quantum Revolution decoders, it is a very handy tool. Tony's sells it for $90. What's really good about it, is you can not only alter the sound file, but also use QSI's CV manager to completely set up all parameters of the decoder. This is especially useful for some stuff like the BEMF setting which are a bit confusing to set up right. When they are, slow speed operation is great.
I will also work with JMRI Decoder Pro, but will onlt Read and Write to QSI decoders. It can Write to non QSI, but I haven't yet had another brand read back.