Adding silence to audio: gaps at the start, middle or end

Yes. Pick the length and press "At start" or "At end": the silence goes right before the first sample or right after the last one. Pressing "Insert Silence" with the playhead near an edge snaps to it as well.

Yes. Click the waveform where the pause should go (or pause playback at that moment) and press "Insert Silence" or the I key. You can add as many insertions as you need; each one shows in the waveform and in the list below it.

Set the length to 2 seconds and press "At start". The gap appears in the waveform straight away and is audible in the preview, so you hear the pause before you export rather than discovering it is half a second short afterwards. To add a breath between two sentences that ran together, click between them and press "Insert Silence" instead.

The audio between the gaps is not edited. Saving to WAV or FLAC keeps it exactly as it was; MP3, OGG, M4A and OPUS encode it again, so for those pick a bitrate at least as high as the original's.

With Premium, yes: drop several files onto the upload area together, set the silence at the start and at the end, and press the button. Each file keeps its name with "_silence_added" at the end and goes to a folder you choose, or into one ZIP archive. Without Premium the editor takes one file at a time.