Combine the soy sauce and brown sugar. Pour half the dressing into a large bowl and add the salmon. Let marinate for 10 minutes, tossing occasionally. Reserve the remaining dressing.
Heat the oil in a large nonstick skillet over medium heat. Remove the salmon from the marinade and season with ½ teaspoon salt. Cook in the skillet until opaque throughout, 3 to 4 minutes per side. Break into large pieces.
Combine the mayonnaise and Sriracha.
Divide the Sriracha mayonnaise, salmon, cucumbers, cabbage, and scallions among the tortillas and drizzle with the reserved dressing.