Could I add/edit and synchronize the lyrics of the mission through the mobile apps (Android/iOS) ?

No, the only way to complete your task is working by the desktop app.
ATTENTION: the lyrics not synchronized via Desktop app will be not considered as completed

I'm not sure about a lyrics. What should I do?

When you have a little or big doubt about a lyrics (strange word, difficult understanding, regional language, etc), you must delete the task from your list. The task will be reassigned to another freelancer.

For deleting the song, you'd tap on the trash symbol at the right of the Edit button and choose "Cancel task".

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A song is sung in two languages (e.g. Hindi and English), what should I do?

When a lyrics is a mix of 2 (or more) languages, you'd transcribe it in both languages.

Song example

Fight fight, fight like Bruce Lee
Fight fight, fight like Bruce Lee
Fight fight, fight like Bruce Lee
Fight fight, fight like Bruce Lee

मेरा सोला का डोला
छियालीस की छाती
सीधी बात बोलूं
बातें घुमानी नहीं आती
आता माझी सटकली

The song that I'm listening is instrumental. What should I do?

You'd tap on the 3 dots in the center of the screen and select "Mark as instrumental". The lyrics will be consider as "Completed".

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