So it was a glitch on my system. I don't know how this happened, soundtouch was already installed because of gstreamer bad plugins package. Anyway I am glad to hear that it was just my local problem. I just installed it the regular way, see below.
$ sudo pacman -Sy audacity
:: Synchronizing package databases...
core is up to date
extra is up to date
community is up to date
multilib is up to date
resolving dependencies...
looking for conflicting packages...
Packages (13) blas-3.6.0-4 cblas-3.6.0-4 lapack-3.6.0-4 lilv-0.22.0-2
lv2-1.12.0-1 portsmf-228-2 python2-numpy-1.11.0-1 sbsms-2.0.2-2
serd-0.22.0-1 sord-0.14.0-1 sratom-0.4.6-1 vamp-plugin-sdk-2.6-1
audacity-2.1.2-2
Total Installed Size: 44.47 MiB
:: Proceed with installation? [Y/n]
(13/13) checking keys in keyring [#########################] 100%
(13/13) checking package integrity [#########################] 100%
(13/13) loading package files [#########################] 100%
(13/13) checking for file conflicts [#########################] 100%
(13/13) checking available disk space [#########################] 100%
:: Processing package changes...
( 1/13) installing blas [#########################] 100%
( 2/13) installing lapack [#########################] 100%
( 3/13) installing cblas [#########################] 100%
( 4/13) installing python2-numpy [#########################] 100%
Optional dependencies for python2-numpy
python2-nose: testsuite
atlas-lapack: for improved performance of some linear algebra operations
( 5/13) installing lv2 [#########################] 100%
Optional dependencies for lv2
libsndfile: Example sampler [installed]
gtk2: Example sampler [installed]
python2: Scripts [installed]
( 6/13) installing serd [#########################] 100%
( 7/13) installing sord [#########################] 100%
( 8/13) installing sratom [#########################] 100%
( 9/13) installing lilv [#########################] 100%
Optional dependencies for lilv
bash-completion
(10/13) installing vamp-plugin-sdk [#########################] 100%
(11/13) installing sbsms [#########################] 100%
(12/13) installing portsmf [#########################] 100%
(13/13) installing audacity [#########################] 100%
:: Running post-transaction hooks...
(1/1) Descriptions