Auto Recorder PRO v1.3.8

Auto Recorder, starts and stops the record function when sound of a sufficient level is picked up by the microphone. This feature saves valuable recording time by not recording periods when there is little or no sound.

No ADS.
“Auto upload” to Dropbox option.
Can record when the application starts.
Can record when the device turns on.

PRO:

Android Wear support
Smart Voice-activated Spy recording.
Mic calibration recorder.
Stopping recording automatically.
Live audio spectrum analyzer.
Unlimited recording time.
Date-based naming.
Background recording (You can open other app while recording, even when display is off).
Built-in share memo (Bluetooth, Drive, Dropbox, Email, Gmail, WhatsApp, etc.).
Audio files .wav (3GP, AMR, MP4) export over the USB.
Supports any OS (Linux, Mac OS, Windows, etc.) to playback using VLC.
Landscape and portrait display modes support for Tablet.
Show free space.
Can use the rear microphone.
Can increase microphone volume.
Can delete older records if you are running out of storage space.
Can set the sampling frequency (kHz).
Handle recording by text message.

For control via SMS must first activate the setting, you can use the following SMS:

Auto Recorder Pro REC
Auto Recorder Pro STOP
Auto Recorder Pro threshold 5

The threshold has values between 1 and 10.

In telecommunications, a voice operated switch, also known as VOX or Voice Operated eXchange, is a switch that operates when sound over a certain threshold is detected. It is usually used to turn on a transmitter or recorder when someone speaks and turn it off when they stop speaking. It is used instead of a push-to-talk button on transmitters or to save storage space on recording devices. On cell phones, it is used to save battery life. Intercom systems that use a speaker in a room as both a speaker and a microphone will often use VOX on the main console to switch the audio direction during a conversation. The circuit usually includes a delay between the sound stopping and switching direction, to avoid the circuit turning off during short pauses in speech (From Wikipedia).