On most of the pages i'm in the process of creating, I put a link for download-able music. In my eyes this is the only way to show proof of Metal-ness.

Can this be considered proof?

If not, what is the alternative for me to show proof of metal-ness for all the bands on their page?

Yes, a download link or a link to somewhere that streams music (Facebook/MySpace/ReverbNation/Bandcamp) is the normal way to do it. Other acceptable proof includes a scan of a respected fanzine that describes the music as metal, for bands where the music isn't available.

It's all in the rules: http://www.metal-archives.com/content/rules

"First, for the lesser-known bands, we need compelling evidence that the band is metal. We won't just take your word for it, sorry. The best evidence is of course in the form of audio samples. Most bands have a Myspace with such samples nowadays. But for older, obscure bands (the kind of thrash band that released one demo in 1986 and disbanded, for example), if no sound samples are available, a scan from a metalzine review describing the band's sound as unambiguously metal can be acceptable, but those cases are exceptional and the moderation can exercise full discretion.

Second, for a band to be considered metal by the site, the band must have at least one fully, unambiguously metal album. This means that Def Leppard can be accepted because of their NWOBHM debut, even if they turned to pop rock later. This also means that DevilDriver, which started as mallcore, were (reluctantly) deemed acceptable after finally releasing some metal albums. This does not mean that we'll accept Sum 41 because of their one heavy song "The Bitter End", or that we'll accept Soulfly because of their recent borderline albums."

This part should probably be updated.

Took me a minute to realize that's what the rules say and not a mistake by Zodijackyl...

In some exceptional cases, a link to Mediafire or similar file-sharing service might be necessary. This is useful for bands that don't have much of an online presence, or albums that are so new that any appearance of the songs on Youtube get taken down for copyright reasons. The best way to approach a download link like this is to offer it via PM for the moderators, though this is not the site's official policy just my own method of madness.

More often than not the album can be heard almost entirely through Youtube though.

Oh dear...