UNNotificationSound not playing custom sound

I can't seem to get custom sounds to play on iOS 10, even though the same sounds played on iOS 9 using the old APIs

UNMutableNotificationContent *content = [[UNMutableNotificationContent alloc] init];
content.body = ...
content.categoryIdentifier = ...
content.userInfo = ...
content.badge = ...
UNNotificationSound *bell = [UNNotificationSound soundNamed:soundName];
content.sound = bell;
UNTimeIntervalNotificationTrigger *trigger = [UNTimeIntervalNotificationTrigger triggerWithTimeInterval:[date timeIntervalSinceNow] repeats:NO];
UNNotificationRequest *request = [UNNotificationRequest requestWithIdentifier:[[NSUUID UUID] UUIDString] content:content trigger:trigger];
[[UNUserNotificationCenter currentNotificationCenter] addNotificationRequest:request withCompletionHandler:^(NSError * _Nullable error) {
    NSLog(@"error scheduling %@", error);
}];

I haven't seen an reference to there being an issue in the release notes. Anyone else having this issue?