I've just had a similar problem: a delay allocating 2 large buffers using the "traditional" static sdram allocation, plus the reverb that uses sdram_malloc (altogether far below sdram size).
when i flashed the patch to flash it wouldn't start and i was unable to connect (first thought i bricked the axoloti somehow... ).
now, i replaced the reverb's buffer allocation with
static uint16_t buffer attribute ((section (".sdram")));
and everything runs fine.
i've also tried using sdram_malloc for my delay buffers (one left, one right) and then got weird artifacts on one of the channel. i suspect there are some issues with sdram_malloc.