Aegisub has a fairly advanced, customizable audio mode with both the traditional waveform display as well as an alternative spectrum display. Several different timing modes are available for both normal dialog timing and karaoke timing.
To load an audio file into Aegisub, just go to the Audio menu and press Open audio file. If you have a video file (with an included audio track) already loaded, you can use Open audio from video instead, which obviously will load the audio track from the video file you currently have loaded. You can open any type of audio file that your audio provider can decode (more on this below). In addition, if you simply wish to use the audio display without loading any real audio, you can choose Open 2h30 Blank Audio or Open 2h30 Noise Audio.
If opening audio fails with an error that mentions the audio player, or if playing audio results in no sound, switching to a different audio player may help.
Aegisub normally uses FFMS2 to open audio, which can reliably open audio is nearly any format you can think of. On Windows, Aegisub can also use DirectShow (via Avisynth), which can be faster as it does not have to index the audio files before opening them. However, DirectShow is considerably less reliable, and extremely insane things may happen if you open a file with multiple audio tracks via Avisynth.
Aegisub only supports mono audio. Multichannel audio will be automatically downmixed, but the results may be of poor quality for sources of more than two channels.
If you're loading any audio format that isn't an uncompressed (PCM) Microsoft WAV file, Aegisub needs to decode and cache it first. When loaded, the audio is downmixed to mono (see the audio downmixer option if you want to grab one channel only instead), decompressed to PCM (a.k.a. WAV), and (by default) loaded into a RAM cache. This means that you will need a large amount of RAM to open a long compressed audio file. If your computer doesn't have a lot of RAM, or if you're working with a full-length movie, refer to the audio cache option for instructions on how to make Aegisub use its (slower) hard drive cache instead; or decompress the file to WAV first since Aegisub can read from WAVs directly without need for caching.
The exact amount of memory used for any given audio file can be calculated with the following formula: s = ( b * r * l ) / 8 where s is the amount of memory (in bytes - divide by 1024 to get kB), b is the number of bits per sample (always 16 in the current implementation), r is the sample rate in Hz (usually 48000, or 44100 in some cases), and l is the length of the audio (in seconds).
For example, for a 25 minute audio clip at 48 kHz, you will need (16 * 48000 * 25 * 60)/8 = 144000000 bytes ~= 137 MB.
Loading and decompressing the audio into the cache will take a few seconds; Aegisub will display a progress indicator while loading the audio.