Taking a cue from Siri in iOS 14, Google has added the option to send audio messages using Google Assistant. This means that one can now simply say "Hey Google, send an audio message to...
Every time you send an audio message, Assistant will ask you whether you want to send it over WhatsApp or RCS/Messages. There's also the option to directly send an audio message using WhatsApp by using the voice command "send an audio message on WhatsApp." Once you are done saying your audio message, Google will automatically prompt you to send it.
Google also notes that when one sends an audio message using Assistant, it will also send a transcription of the message, though this did not work for me. As of now, support for Telegram and other messaging apps is missing.
For now, the ability to send audio messages using Google Assistant is available in countries that speak English, and in Portuguese in Brazil. This is a server-side rollout from Google so the feature might not be available to everyone immediately, though the rollout should be fully completed in the coming weeks.