Never heard of this problem before, could be doing what you described in your OP somehow changed the horn volume CV down to zero. Since you have JMRI / Decoder Pro, I'd check it with that. There is a newish version (last spring I think) that works with Windows 10 now. Since QSI decoders use CVs with multiple parts (like "CV57.1" or "CV57.4") it might be holding down F2 and F8 together changed one of those sub-set CVs?
Does the decoder have mulitple horn options? Maybe try another horn option and see if that works. May be doing what you did deleted or muted that particular horn somehow.
Thanks for the suggestions, I previously tried all that's been suggested and it seems it's probably not my fault at all, it's the guts in the beast apparently, QSI decoders from what I found on other sites I guess some praise those decoders and others curse them too...I've been away from model RRing for about the past 5 years, my layout became a parts junk collection table for just about anything I was in a too big hurry to put away where it was supposed to go. But renewed interest I'm back but dam, I'm learning all over again. To think at one time I was on top of this stuff and thought I had a lot of answers back then, especially after restoring some of my older HO locomotives to DCC and with sound, islolating the motors or replacing them with better can motors, we all been there done that. I even still had JMRI and Digitrax PR 3 sound loader I used. I loved decoder pro even just using Bachmann EZ Command but along the way I picked up the NCE Power Cab, having not gone any further yet with a booster, still learning the intricate's of all that can be done with their system. And of course my Bachmann 5 amp booster is worhless with NCE Power Cab. But something happened even with my old JMRI program and JAVA, I believe when I turned it back on, JAVA apparently deleted my old JAVA program that worked with Windows 10 and installed an updated program I'm afraid ain't working nice with JMRI, got to work them bugs out too...but I too remember having problems with QSI sound decoders CV's and BLI locomotives being read and written with JMRI, one in particular BLI locomotive stalled for no apparent reason and would start over again but now wondering it this new "keep alive" would have prevented such stalling and digital breakup's. No, it wasn't dirty track or dirty wheels on the loco, when still other DCC locomotives with sound used the same route and never a probem..must have been like now with the horn/whistle just quit in the decoder, I assume nature of the beast as per suggestions from other sites....also I just recently purchased a Bachmann steam loco with sound, Sound Trax installed decoder, Alco, 2-6-0 and i'm quite impressed with the entire loco with detail and the decoder, I think it's great and for the price of $160 including shipping. Time will tell but as of now running it a month I give it 5 stars, very pleased....having frustrated fun anyhow, keep on modeling, and thanks to those who responded, probably won't do nothing, until the decoder just quit's in the Atlas or BLI locomotives and replace with a proper sound decoder, love the TCS WOW decoders too...
Try info on this site might help if you have not already tired it
https://www.trainorders.com/discussion/read.php?3,3203881 wand and CVs are listed , disregard other comments, and try google search for QSI decoder reset might offer other help.
Ranger,
Did you try activating the horn with either the F2 or the HORN/WHISTLE button? I've had a few sound decoders where the bell wouldn't initially activate when pressing the BELL button on my Power Cab but would after I had activated it with the F1 button. After that both buttons activated the bell.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
I sounded the horn while the loco was in motion running a train but accidently while holding the horn button down I accidently hit the 8 button and it all silenced, hit the 8 button again and all sounds returned except the horn. Tried resetting it with the wand that came with the loco, but that didn't work, tried doing it with the CV's but even that failed. Using NCE Power Cab with no booster, anybody know how to undo what I messed up.....