The length of a ram preview depends entirely on the system resources that you have available weighted against the frame size, resolution and frame rate of your composition. AE is not a NLE and in it's current version you cannot expect full rez 4K previews that last more than a couple of seconds, full rez HD playbacks will be under a minute with a pretty healthy system and SD video may give you a minute or two of preview. You just have to learn how to know what you are getting by looking at the pieces of your project or you have to render your shots and cut them in a NLE like Premiere Pro.
The problems with your audio could be related to the format of the audio in the source file or some kind of OS/AE version resource error the causes things to go haywire.
Here'a a suggestion for getting longer ram previews with smooth audio. First, use .wav files that comply to the video standard of 49 HKZ 16 bit PCM stereo or mono in your project. Second, create your compositions using the standard HD presets in the Composition Settings panel. 1920 X 1080 29.97 in NTSC land (USA) will give you the most success. Set your Magnification Ratio, first option on the bottom left corner of the composition panel, to 50% and set the Comp Panel resolution to Auto. Depending on your version of AE you'll either use the 0 key on the numeric keypad or on the latest version the space bar or for all versions you can use the menu and choose Composition>Preview or use the Preview Panel.
If you still do not get good sound playback or your playback is too short then select skip 1 frame in the Preview Panel. This should give you something usable as a preview of your shot. If you still want a longer preview set the Composition Panel Magnification Ratio to 25%.
One last point. AE is not an editor, it's an app designed to create visual effects or motion graphics shots or short sequences. More than 90% of my AE comps are less than 10 seconds long. How long as it been since you have seen a 10 second shot in a movie? Trying to cut a video that is longer than just a few seconds on AE is excruciatingly complex and inefficient. Create your shots in AE and edit them in Premiere Pro.
I hope this helps. If you are new you'll also need to learn about rendering, video formats, audio standards, compression and the limitations that frame rate choices make on the kind of motion you can create. Here's a good place to start: Basic Workflow
Actually,before i do anything,i want to point out that i've made videos like i want to make now before,but all of a sudden it's like this.Is the software problem or?