Barcode API has a "DecodeSound" property that should do exactly what you need... but is not supported on Android...
I had a partner doing something similar on TC55 and MC40. If I remember correctly, they set the "decodeVolume" property to '0' and then used the mediaPlayer API to play the wav file they wanted in the barcode callback.
It's a workaround, but it works if you can live with a bit of additional latency.
According to the EB dev team, there were two issues regarding barcode scanning sounds:
1. The default beep sound upon scanning with TC70 (GA1) devices was not working when invoked with 2.2 APIs. This was fixed in EB 1.3 (and higher).
2. Customizing the sound upon scanning (i.e. decodeVolume, decodeFrequency, decodeDuration) does NOT work on KitKat devices because of a limitation in the common API in EMDK.
Please let us know if and how you might have worked around the issue.