What does it mean for audio to be 'sped up'? How can I undo it?
It could mean:


That the metadata in the .wav file wrongly indicates that the signal was recorded at a sample rate 10x higher than the true value (for example it is advertised as having been recorded at 80kHz while it has been recorded at 8kHz). Simply fixing the metadata undoes this.
That the signal has been downsampled by a factor of 10, but is still tagged as having been recorded at the original sample rate. This can be undone by dividing by 10 the playback sample rate (by tweaking the metadata); or by upsampling the signal by a factor of 10 without changing the playback sample rate. Note that in any case, 90% of the bandwidth of the original signal (the high frequency content) has been lost and cannot be restored (or only very approximately through spectral band replication).
That a time-stretching (time-scale modification) algorithm has been applied to it. This is a lossy process, and cannot be undone without severe loss at such a high stretch factor. The "antidote" is simply to use any time-stretching algorithm with a stretch factor of 10x.