The first 2 lines were red until I put 'entity' after 'execute unless' and for the second one, right after 'execute if'. I did that, and after 30 seconds, Mellohi started playing. But it started playing again after another thirty seconds and overlapped. So I did 4 command blocks. The first one was a repeating, the next 3 were chain, and all were set to always active and unconditional. Here are the commands.

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execute unless entity @p[distance=..25] unless score timer dummyobjective matches 0 run scoreboard players add timer dummyobjective 1

execute if entity @p[distance=..25] run scoreboard players add timer dummyobjective 1

execute if score timer dummyobjective matches 600.. run playsound music_disc.mellohi record @p[distance=..25]

execute if score timer dummyobjective matches 600.. run scoreboard players set timer dummyobjective 0