[PinkMilk]

That is because you replaced the

{param}

with a static file,

{param}

is needed so to be used multiple times like you had originally so when you or a member posts:

[videobb]http://mydomain.com/mp4/file.mp4[/videobbc]

it will show up and

[videobb]http://mydomain.com/mp4/file1.mp4[/videobbc]
[videobb]http://mydomain.com/mp4/file2.mp4[/videobbc]

would show 2 different videos.

{param}

is what gets "swapped out" and replaced with whatever you or your members put between the bbcode.

Security/abuse note:
If you allowing members to upload files to your server for posting or even if it's just you I would suggest adding part of the url within the replacement code:

<source src="http://yourdomain/mp4/{param}.mp4" type="video/mp4">

then making them just add the name of the file, so what you or they would post is just:

[videobb]file1[/videobbc]

otherwise anybody/spammers etc could take advantage and post what ever video they liked for instance:

[videobb]http://reallynastyporn.com/girl/gets/tickled.mp4[/videobbc]