Apologies for the oversight there, looks like you’ll also need to null out the other exinfo callbacks:
dialogueSoundInfo.exinfo.pcmreadcallback = null;
dialogueSoundInfo.exinfo.pcmsetposcallback = null;
dialogueSoundInfo.exinfo.nonblockcallback = null;
dialogueSoundInfo.exinfo.fileuseropen = null;
dialogueSoundInfo.exinfo.fileuserclose = null;
dialogueSoundInfo.exinfo.fileuserread = null;
dialogueSoundInfo.exinfo.fileuserseek = null;
dialogueSoundInfo.exinfo.fileuserasyncread = null;
dialogueSoundInfo.exinfo.fileuserasynccancel = null;