What MIDI Synth are you using? The built in Microsoft GW Wavetable or some other MIDI Synth?
The problem would appear to be in the MIDI Synth you are using rather than the sound card, as the sound card only takes whatever the MIDI synth sends to it and converts that to audio.
If you are using, for example Band in a Box which generates songs via MIDI, the old Roland GS MIDI synth is incompatible with Win 7 and they are supplying a new, Win 7 compatible, MIDI synth with the latest version.
If you are using a MIDI synth other than the Microsoft, check with the MIDI synth vendor for any Win 7 updates, since it apparently worked OK before you upgraded to Win 7.