In 2E AD&D, ranges for missiles and spells are by default given in yards. Does this ruleset ever specify a different conversion when used indoors/underground? Citation/reference desired.
No, not for missile or spell ranges
According to the AD&D 2e (Revised) Player's Handbook, missile and spell ranges have a set range in yards (PHB p.131, under Range):
The first step in making a missile attack is to find the range from the attacker to the target. This is measured in yards from one point to the other. This distance is compared to the range categories for the weapon used (see Table 45 in Chapter 6: Money and Equipment).
The table in question (PHB p.95) specifies "Range (Yards)" and includes a summary of the rules, which makes no mention of any difference between dungeon and overland. There's no mention of a difference in the full missile weapons rules on p.131 either.
Likewise, the rules on spell range/area of effect (PHB p.168) say only that the spell specifies the range, and each spell has its own range specified in specific units, typically yards for range (e.g. magic missile, p.176, has a range of 60 yards + 10 yards per level).
AD&D 2e only changes scale for movement, not range. In contrast to range, which specifies a unit, characters have a movement rate as a flat number, which must be multiplied by a scale of feet or yards.1 This doesn't apply to missile/spell range; yards are still yards when you're in a dungeon, you just walk more slowly indoors because, canonically, you're being cautious and paying attention to your surroundings.
1. For the benefit of future readers finding this answer, that scale is 10x feet per round in a dungeon or combat (AD&D 2e revised PHB 128, 157), 10x yards per round otherwise (PHB 157) or 2x miles per 10-hour day overland (PHB 158).