Update inside (Insert) Trigger and Unique constraint...

I'm trying to build a table that will store a history of records
by enumerating the records. I want the newest record to always
be number ZERO, so I created a trigger on my table to handle the
assignment of version numbers:

CREATE TRIGGER "trg_audio_file_insert" BEFORE INSERT
ON "public"."audio_file" FOR EACH ROW
EXECUTE PROCEDURE "public"."trg_audio_file_insert"();

My trigger function looks like this...

CREATE FUNCTION "public"."trg_audio_file_insert" () RETURNS trigger AS'
BEGIN
...
/* rollback the version number of previous versions of this
audio_id */
UPDATE audio_file SET
afile_version = afile_version + 1
WHERE acct_id = NEW.acct_id
AND audio_id = NEW.audio_id;

/* newly inserted row is always the latest version ''0'' */
NEW.afile_version := 0;

...
/* yeah, that worked */
RETURN NEW;
END;
'LANGUAGE 'plpgsql';

There exists a unique constraint on the 'the audio_id / audio_version'
columns. However, when I insert records into this table, I'm getting an
error like:

duplicate key violates unique constraint "idx_audio_file_id_version"
CONTEXT: PL/pgSQL function "trg_audio_file_insert" line 18 at SQL
statement

I don't understand WHY there could be a violation of the constraint when
I clearly asked for the update to be performed prior to the assigning of
NEW.afile_version := 0;. Yes, there exist two records with my acct_id and
audio_id with versions 0 and 1 already. The update should roll them to
1 and 2 then the insert at 0 should be unique still.

Why isn't this working? What's the deal with ordering when it comes to
triggers? Is the update not performed when I tell it to?

Dante