Regarding ionic select in v3 I used the below code where we have (ionSelect) in ion-option to fire select option event.
<ion-option selected="true">Standard tone</ion-option>
<ion-option *ngFor="let tone of ringtones;" [value]="tone.Url" (ionSelect)="playSound($event)">{{tone.Name}}</ion-option>
</ion-select>
Let me know alternate in ionic 4 since in v4 not supporting `(ionSelect)`