This is a defect in 13.3 CLRT-72889.
Assignment changes are being logged in to audit tables but UI does not show it. This defect is fixed in 14.1. I tried your suggestion and it did not make any difference,
select t.prname Task, (select full_name from srm_resources where id=a.prresourceid) res, u.column_name, u.value_before, u.value_after,u.created_date
from cmn_audits u, prassignment a, prtask t
where u.object_code='assignment' and u.object_id = a.prid
and a.prtaskid=t.prid and t.prprojectid=5118257
and u.created_Date > '01-JAN-2015'
My question is how to create a query based portlet to be available on the project tab with parameter to filter by that specific project.