Update Unity has add Force iOS Speakers in PlayerSetting, you would try this.

WARNING

This is based on https://github.com/10people/UnitySpeakerFix, I have update code to use on iOS 15.0 with Unity 2021.2.17f1 (Apr, 2022)

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Unity iPhone Speaker Fix

When using microphone input, Unity iPhone switches audio output to the earpiece speaker, instead of the usual speakerphone speaker. As a result, all the audio in game suddenly becomes very quiet, even at max volume. In particular, this is problematic for games which include voice-chat.

How-To

Simply install the Unity package, add a call to iPhoneSpeaker.CheckiOSPrepare() before you call Microphone.Start(). add a call to iPhoneSpeaker.ForceToSpeaker() before you call audio.Play().

Credit

Base code from these two SO threads:

• http://stackoverflow.com/questions/9784573/play-and-record-audiosessionsetproperty
• http://stackoverflow.com/questions/5036399/how-do-you-force-audio-to-come-out-of-the-iphone-speaker-when-a-headset-is-attac