When I create a new song in NWC, the first 4 staves always has a piano sound, but when I add more staves, these staves sounds as 'ping' when I add notes to that stave. Even when changing the instrument to another, it is sounds unchanged. Also when saved to nwctxt, that file does not contain instrument names.
What is the best way to add staves which has the 'basic' piano sound?
I would recommend starting with the sample files. Use File, Open Sample to try them. The Jingle.nwc sample specifies a piano sound for both staff parts from Staff, Staff Properties.
Each time you add a staff, NWC will assign the next available MIDI channel to the new staff, since you might want unique instruments to be on each staff. It sounds like your 'ping' staff might be assigned to channel 10, meaning that you are hearing General MIDI drum sounds (which are automatically produced for channel 10).
If this is the case, try going into staff properties and changing the channel number of the offending staff to something other than 10. If you have multiple staves that you know will be playing the same instrument (i.e. if they are layered for display purposes), you can safely assign them the same MIDI channel.