Midi Playback

[stp]

I got a MIDI from a online shop. When making it to playback trough MIDI , every staff sounds after the instrument it is intended to (violin, trumpet,..) through the Microsoft Synth... but the instrument list is pointing, in all cases, to Acoustic Grand Piano (GM 1). I suppose there is a place (possibly in the beginning of each staff that changes patches, or something like that

May I know where is the right place to check?

thanks