ROCK/POP Dire Straits: ''Alchemy.'' (Warner Bros. 25085-1 G.) - It's nice to hear a double live album that so fully realizes the promise of its studio forerunners. The little that's lost on ''Alchemy'' to the clamor of the concert hall is more than counterbalanced by the freedom live performance breathes into Dire Straits's sometimes slightly cramped sound. Of course, Mark Knopfler's vocals still sound like confidential asides that have been amplified. Yet his gruff, almost completely flat voice - especially here - adds an appropriate sense of irony to his songs. To be sure, ''Alchemy'' doesn't offer quite the instant pop gratification of, say, a Boy George hit. Several of the songs last 10 to 14 minutes - and take the time to elaborate on Knopfler's colorful themes. Although there's a certain somberness inherent in most Dire Straits songs, the selections here vary considerably in format - from the quick-tempoed ''Sultans of Swing,'' for instance, to the reflective ''Telegraph Road.'' Throughout these cuts, splendid guitars and keyboards work overtime to draw in shadows and colors, and etch character lines.