Caveat: I know nothing about Solaris audit files, but since there are no answers yet...

dd(1) is a useful tool for dealing with binary data
Are the records in the binary file a fixed size?
If so, and you have an idea how many records you want to copy from the original file, you could do something like

Code:

dd bs=record size count=n if=input file of=1st output file

to copy n records from the beginning of the file. Then to copy the remaining m records

Code:

dd bs=record size skip=n count=m if=input file of=2nd output file