It might be possible to use the Sound.duration property to detect the number of loops, although avoiding timing problems would be tricky.

I think it's not just an testcase issue anymore.
I suspect that some flash game seemed to be affected by this too.

UFO Joe (Flash 6)
http://www.miniclip.com/games/ufo-joe/en/
(http://www.miniclip.com/games/ufo-joe/en/ufojoe_flash8.swf)
In this game, intro sound on each level will be looped, and if you get into landing or crashing, you will get their sound effect looped too.

Pest Attack (Flash 7)
http://www.madoo.com/games/PestAttack/
(http://www.madoo.com/games/PestAttack/index.swf)
In this game, when you swatted rat, swatting sound will be looped, if you swatted more than one time, it will sounded like a chaos.

I recreated this from git master.

What is expected behavior for repeat 0, -1, and -2?

For 0 I imagine you expect Flash's behavior: repeat once.

Hello, I have used Gnash 0.8.9 and Gnash 0.8.10 to play flash7 file that tries to invoke sound within ActionScript 2.0, by using this code

snd=new Sound(this);
snd.attachSound("beep-note");

trace("sound: starting (repeat 0)");
snd.start(0.0,0);

It supposed to play sound from offset '0.0' with repeat value '0' which Flash Player will treat it like a repeat value '1' (aka. play the sound once)

But on Gnash, it was treated as an infinite-looped sound, which was an incorrect behavior.

http://www.mediafire.com/download.php?v94sl7efynp80lf
This is a 'correct' result from Macromedia Flash Player 7
You will notice that sound was played only once.

http://www.mediafire.com/download.php?62p0gt95r6151od
This is a video result from Gnash 0.8.10
You will notice that sound was looped infinitely, and there was also a gap between loop, which seems to be a symptom of bug #24481 (ActionScript sound loop gap)

http://www.mediafire.com/download.php?5h7gl9zqyrv4z47
These are my dummy flash files (created from Flash MX 2004), accompanied with source code, and also attached here.
(These videos was recorded using soundrepeat.0.swf [SoundRepeat0.as], but other files will behave the same)


Note #1: both zero and negative repeat value was treated by Macromedia Flash Player as it was '1'.

Note #2: both zero and negative repeat value was treated by Gnash as it was an infinite repeat (any negative value will get the same video result as above)

Gnash: gnash_0.8.9-1~lenny and gnash_0.8.10-1~lenny from getgnash.org APT repository (both yield exactly same result)
System: Debian GNU/Linux 5.0 Lenny on i386