If the leader has not joined the meeting or if you are the only person who has dialled in, hold music will be played until others join the call.
As leader, you can choose if you wish the meeting to start only when you join or to start immediately when two attendees join, even when you are not on the call.
If you are a leader and you can only hear hold music, then the account may be set as 'leader is required', therefore you may need to re-join and enter your leader pin for the meeting to start.
You can change this setting from your meeting dashboard, highlighted below.
Settings>Security>meeting Leader Requirements