I noticed the same problem as @denver. All the audio notes from the device with Android is displayed as loading waveform (recording by a built-in microphone in Signal). The issue does not affect in audio files sent from Android as an attachment (selected from system files) or voice notes sent from Signal Desktop.

To sum up, the problem concerns only the recordings from the built-in microphone with Signal Android, regardless of whether they are sent directly after the recording in the conversation window, or saved and re-sent as an attachment.