kwave

Kwave makes it simple to edit audio in many ways.

Adjust volume level:

Fade in, fade out, or custom amplitude curve
Scale volume by a constant factor
Normalise volume
Adjust frequency content:

Lowpass filter
Bandpass filter
Notch filter
Pitch shift
Other features:

Add noise
Add silence
Convert audio sample rate
Go to position
Insert audio from clipboard
Reverse audio
Save audio blocks between labels as separate files
Sonagram (spectrogram)
Supported file formats:

WAV (*.wav)
Ogg/Opus (*.opus)
Ogg/Vorbis (*.ogg)
MP3 (*.mp3)
FLAC (*.flac)
ASCII coded audio (*.ascii)
all file formats supported by libaudiofile (some formats are import only)