I assume you are asking about Cisco WebEx Meetings Server (CWMS) and not WebEx SaaS. CWMS has two places where you can configure Meeting entry announcements:
1. Global setting for the system (this is applied by default to all the user profiles): Settings > Audio > Telephony entry and exit tone
2. Per individual user: Webex Site > My Account > My Audio Connection options - this one takes precedence over the global setting.
Now, you can't configure this preference on per meeting basis. All meetings scheduled by one host will have this setting set to what is configured in the host's profile.
If you schedule a recurrent meeting when your profile is configured to use Beep, all the meetings in the series will have Beep as the announcement. Even if the host changes this setting in the profile, already scheduled meetings will have the old setting. The only way to change it is to reschedule the series with a new setting configured in the host's profile.
I hope this helps.