You "modified them with a sound card' of what type? Considering that, it almost doesn't matter what its in. I.d. sound issues by sound board and whatever is triggering it in the loco, another board, tach wheel, Reed switch, etc.
Do the bells shut off at speed? No to low speed bells is prototypical in ways. This may be happening by design. Not a perfect design without an on/off, but I could easily see this being a feature in a low cost setup.
Also try activating and holding the bell control on for various times 2 to ten seconds each try. You may have it "locked on" like tmcc.
Check can the transformer simply by switching rail input. If the whistle/horn is now stuck, your transformer has a DC bleed.
It could also be a sensitive trigger circuit and a bi polar cap on the motor and or input power to sound might be needing a filter(cap) too. The "magic light bulb" trick might even work to smooth things.(12v, 14v, 18v bulb powered by track. Experiment some.)
