All functions returns int (or some sort of signed value). If this value is negative it represents error code. Codes to SND_ERROR_BEGIN (500000) represents standard system error. Codes equal or greather than this value represents sound library API error. All error codes begins with prefix SND_ERROR_.
This error is caused if driver uses uncompatible kernel API for this interface and library doesn't know how this API can be used.
This functions converts error code to string. Functionality is same as strerror function from standard C library, but this function returns correct string for sound error codes, too.