Hi Beat,
I'm not finding a specific setting to control the ringtone in the iOS MDM policy.
You may be able to find what you need in the Notifications section of the policy - see here :
https://www.ibm.com/support/knowledgecenter/SS8H2S/com.ibm.mc.doc/pag_source/concepts/mdm_policy_gde_ios_notifications.htm
Best regards,
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DAVID O'GRADY
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