How can I make the audit system create new audit files if they already exist?
There doesn't seem to be any reference to being able to directly manipulate the audit file names in 10g.  The closest thing I found is Bug 6023472 which lists the following changes introduced in 10.2.0.5/11.1.07/11.2.0.1:


OS/XML Audit filenames will have a filename format of the form:
  &lt;InstanceName&gt;_&lt;ProcessName&gt;_&lt;ProcessID&gt;_&lt;SerialNo&gt;.&lt;Extension&gt;
The default value for Max Audit FileSize is 1000KB and Max Audit File Age is 1 Day. This means that once one of these max limits are 
reached, the audit records will be written in a new audit file with same 
"&lt;InstanceName&gt;_&lt;ProcessName&gt;_&lt;ProcessID&gt;_" part, but with a new "&lt;SerialNo&gt;".


I'm not sure how Oracle assigns PIDs, but you could try increasing your PROCESSES parameter to see if that keeps it from re-using the values as quickly.