What is the custom or rule for Sephardim to travel over the three weeks? More generally, are there any restrictions for Sephardim during the three weeks?
Traveling is not a problem. Just stay safe.
Sephardim dont have the 3 weeks, they only have the 9 days and Shavua sechal bo.
What about music?
Only live music
Can a video of someone singing with music be watched during 3 weeks for ashkenazim?
Yes.