If I have Spellsinger and Guardian Angel attached to my Sand Soldier, and the Soldier dies, what equipment will be destroyed?

Only Guardian Angel is destroyed. Svellsongur stays attached and returns to base with the soldier.

Here's the breakdown:

1. The death event triggers the replacement effect. Guardian Angel reads: "If I would die, kill Guardian Angel instead. Heal me, exhaust me, and recall me." The "I" here refers to the equipped unit (the Sand Soldier). When the soldier would die, Guardian Angel replaces that event — Guardian Angel itself is killed, and the soldier is healed, exhausted, and recalled to base.

2. Svellsongur copies the unit's text, not other gear's text. Svellsongur says: "Copy that unit's text to this Equipment's effect text." A Sand Soldier token has no rules text (it's just 2 Might with the Shurima tag per Rule 184.3), so Svellsongur gains nothing from it. It has no triggered ability of its own to fire.

3. The soldier never "dies." Because Guardian Angel's replacement effect prevents the death (Rule 370.1), the soldier stays on the board and merely moves to base. Per FAQ #9097, #8298, and #5370, when a unit is saved by Guardian Angel, all other equipment remains attached. Svellsongur stays equipped and recalls to base with the soldier.

Result: Guardian Angel → killed (trash). Svellsongur → still attached to the soldier at base.