node_load() Performance and alternatives
First i would make sure that using node_load() does slower your script. If you are sure it really does you can either:


1- Create an index_audio  table with a nid and use_audio columns.
You could update this table on node creation/update/delete. Then
you can just query this table to make sure a specific node does have
audio attached to it. I would use that solution only if i you have to
intensively make such a check
2- Directly query your field table with your nid as etid, in your
instance : field_data_field_efe_audio table see if it returns
FALSE or some actual data