Hi, yes Rockbox will void your warranty.
From the Rockbox website :
Does it void my warranty if I use Rockbox?
Yes.
However, most large companies tend to be lenient with warranty claims.
So even if using Rockbox will technically void your warranty, quite
probably the aftersale service won't cause any trouble. Of course, they
can, but it is not really in their "commercial interest" to do so, and
there is plenty of testimony from users who got their device replaced
even if the issue was caused by misuse (example: hard drive dead due to
a fall, or even LCD screen broken).
Iriver now have the following statement in their online
warranty terms, specifically naming Rockbox as voiding the warranty:
"This warranty is void if [...] the player is opened, repaired,
modified or altered by third parties e.g. Rockbox, unless such repair,
modification or alteration is expressly authorized in writing by
iriver". Archos has also been known to refuse warranty repairs for
people that have returned their units with Rockbox flashed.
On the Sandisk Forums :
Rockbox is neither created nor endorsed by SanDisk. Any
"bricking" or malfunctioning of the device (while trying to install
Rockbox) can be deemed irreplaceable. We are aware that users like to
have options, and as this seems to be a popular alternative. So as a
courtesy to our community members this has been stickied
I tried it and it's not really hard to install or anything, and can also be uninstalled easily. But in the end, I prefer using the original Sansa Firmware.
Also, note that Sandisk gave informations to Rockbox devs about how the player works... so, if you install it, and get a problem with something else, they might still replace it under warranty.
Blarg.